I don't think the question is really about whether AI art is real art. (But it could be about that, as I'm not familiar with commercial cons in the US.)

Some years ago, around the time I became aware that AI art is a thing, the artist scene around Finnish cons had already decided to ban it. And the reason was obvious, as the same people are also very eager to police others who might be selling pirated products.

They don't care legal constructs such as intellectual property. They don't really care about economic constructs such as copyright. What they care about are authors' moral rights. If the model was trained without obtaining a permission from the authors of every work in the training data, they think using the model to create art is immoral.