I fully realize that these words and other similar ones, can be used in both a legal and very common way. The problem with using these words in a public, online context is that the words, if used incorrectly, may be the basis for a defamation action on the part of the person accused of "stealing" or of "theft."
I guess I should first address the issue of whether or not a violation of the copyright law can be considered a theft of personal property. The United States copyright law is found in chapters 1 through 8 and 10 through 12 of Title 17 of the United States Code. I think it important to point out that the terms "theft," and "steal" do not appear anywhere in the Copyright Law of the United States. With regard to use of the term "theft" in conjunction with copyright infringement, here is a quote from the Wikipedia article "Copyright infringement."
Copyright holders frequently refer to copyright infringement as theft. In copyright law, infringement does not refer to theft of physical objects that take away the owner's possession, but an instance where a person exercises one of the exclusive rights of the copyright holder without authorization. Courts have distinguished between copyright infringement and theft. For instance, the United States Supreme Court held in Dowling v. United States (1985) that bootleg phonorecords did not constitute stolen property. Instead, "interference with copyright does not easily equate with theft, conversion, or fraud. The Copyright Act even employs a separate term of art to define one who misappropriates a copyright: '[...] an infringer of the copyright.'" The court said that in the case of copyright infringement, the province guaranteed to the copyright holder by copyright law—certain exclusive rights—is invaded, but no control, physical or otherwise, is taken over the copyright, nor is the copyright holder wholly deprived of using the copyrighted work or exercising the exclusive rights held.I intentionally left in all the links and links to footnotes in the quote. From my own perspective, I think it would a lot easier to prove a claim of defamation in this instance, than it would be to prosecute and prove a case of copyright infringement. I would suggest that if you are going to use any of these emotionally laden words in accusing someone of copyright infringement, you should think more than twice. In most, if not all states, theft and stealing are criminal offenses.
Now accusing someone in the context of a copyright infringement of piracy is quite a bit different. Piracy is a term used to refer to the infringement that involves the sale of copies of the protected work. Use of the term piracy in a copyright context, pre-dates the implementation of any of the iterations of the U.S. Copyright Law. However, in the context of U.S. Copyright law, piracy does not equate with either theft or stealing. Obviously, there are some instances where the genealogical work is actually covered by copyright and produced for a profit. In this case, the word "piracy" might not be objectionable.
But I would still be extremely cautious in using any of these terms in conjunction with what you believe to be copyright infringement. Even if you have a valid claim for copyright, you do not want to muddy the water in your claims with a counterclaim for defamation. The current legal action for defamation includes both of the old common law actions for libel and slander although some states still retain separate actions for libel and slander. The essence of a defamation claim is that a person's reputation has been harmed by either a written or oral statement that is false. Calling someone a thief or accusing someone of stealing in a copyright infringement case may be per se actionable since neither term applies to copyright infringement.
Defamation per se is an action where the courts have recognized that some types of statements can be considered harmful without the plaintiff (the defamed person) being required to prove damages. One of the categories of such actions is a false accusation of a criminal offense. However, many states in the U.S. have abandoned this older view of the law and now require the proof of actual damages to maintain a claim for defamation. For a discussion of the jurisdictions that now require proving damages, see the American Bar Association article entitled, "Defamation Per Se: Be Prepared to Plead (and Prove!) Actual Damages." One question you need to ask yourself, before using any of these accusatory terms, is whether or not you want to take the chance that the person you accuse will not be damaged by your accusations.
I strongly suggest that credible and responsible genealogists be extremely cautious in throwing around accusations of theft or stealing. If you have a credible copyright infringement claim be absolutely certain of your claim before you go public and start accusing people online. It is legally dangerous to employ terms such as theft and stealing unless you want to become involved in a lawsuit aimed at you rather than enforcing your own copyright claim. Before using either term, I would suggest that you carefully read your state's criminal code definition of theft. In Utah, for example, that code provision is in Title 76, Chapter 6, Part 4, Section 401. You may think that the inclusion of terms such as "intangible personal property" or other such wording in the statutes will apply to copyright. But you must remember that copyright claims are based on Federal Statutes.
Let's suppose that in your jurisdiction that you are certain that copyright infringement is actually considered to be theft so you accuse your opponent of theft. That would still not prevent a defamation counterclaim in any action you took to enforce your copyright claim. If you ultimately lost your claim and the court decided you did not have a proper claim for copyright infringement, you could still remain liable for defamation. Accusing people, either with or without foundation, is just not worth the risk.
In the context of this post, I am not addressing any particular claim or injury from either side of the issues. If you believe your copyright has been infringed, you should consult a licensed intellectual property law attorney in the place where you live. Please see my Disclosures and Disclaimers page for further information.