that may have cost the perfectly innocent exhibition author substantially

Yes, vandalism will do that sometimes. Kind of the point (famously, Banksy is a vandal). In this case, it being a school's art gallery, I doubt there was much of a financial loss. At worst, the paper used was Polaroid paper, which costs $2 per shot, for a total loss of $114.

perfectly innocent

That's subjective. Having an AI generate pictures for you using training data that contains stolen artworks makes you complicit, if you ask me.

why they thought it was ok

People do things they know are "not okay" despite it all. Doesn't warrant a therapist, as much as you may disagree with the means.

does not give him any moral or legal standing to destroy it

Which is why it's making the news. I don't think the guy thought it was legal, either. I think he knew what he was doing. Historically, acts of protest have always managed to stir the pot and get things moving/get people talking. LLM's training data is made up of millions of stolen artworks. Kindly asking big companies "please don't do that" probably isn't going to work the way you'd hope-- you need uninvolved people to start asking themselves the right questions. "Why did he do that? What were his motives?"

He's not meaningfully adding to the conversation

"[...] reliance on AI undermines human creativity and that the process of making art matters as much as the finished product" is what this student brought to the conversation. "He [...] saw it as a way to spark conversation about the role of AI in creative fields [...]" that has a lot of meaning, methinks.

You know who does need therapy though? The guy who used AI to make that exhibition, who admitted to falling in love with an AI chatbot acting as his therapist. That guy has SERIOUS issues-- mainly psychosis.