Or this hypothetical. Is abusing the GPL wrong? If so, why is that wrong and piracy is okay? At a high level, it's the exact same issue - someone says "I've produced this intellectual property, I want other people to do this or not do that with it", and someone else says "too bad, I'm going to do what I want. Deal with it".
Illegally appropriating GPL code ISN'T THEFT EITHER.
Theft is a very specific thing. Copyright infringement is another, but different, very specific thing.
Whether or not you believe them morally identical isn't relevant. Your assumption that people complaining that copyright infringement isn't theft are thereby excusing copyright infringement is flawed. That discussion is completely orthogonal.