The argument goes that stealing involves taking something from someone, whearas pirating produces a copy. If you take someone's car, they don't have a car anymore, but copy their game and now you both have one.

That said, I don't think it follows from "buying isn't owning" that "piracy isn't stealing". Its a pithy slogan that captures a mood, but I don't think it really makes sense.

Your position, I believe, is more coherent: "I don't care if it is stealing". It might not technically be stealing, but its a breach of the terms of the sale that you tacitly agreed to when buying.

Something similar can be said for adblocking. People argue that it is "stealing" because there is a breach of the terms of use, and the ads fund the content. It might not really technically be stealing, but it is simpler to just say "I don't care what it is, I don't feel bad for giant corporations". You can also say this about shoplifting.