Don't underestimate piquing interest. Indeed, that was the real reason behind having the galleries online in the first place. It wasn't to give people art to use in a game (although it certainly did that). The art, mixed with an inside look at a class, a few feats, a location, and a few monsters, were there to get people to buy physical books. Now, moving the art and the classes behind a wall of subscription shows to me that it's not as much about the books anymore, as it's about getting people onto a subscription.
That's fine and dandy. That's not me. I'm not that target demographic.
As a note, they still have many more excerpts these days for all their upcoming products (along with the art posted in the previews themselves.) It may well be they feel that stands as a strong enough preview for showing potential customers what they have to offer.
I think the art would certainly be a good supplement to those previews, but I can understand their logic, and one should acknowledge exactly how much upcoming info they share freely these days.