There's another aspect to consider:
When people express themselves in a creative way, they do so so that other people (and themselves!) can appreciate it. We create art for other people - we don't create for animals or plants or robots. Other people can appreciate our art because it may express an emotional and/or psychological state that resonates with them.
Now if we avoid the question of whether or not an AI can be considered creative if it does not express itself voluntarily, how can we judge the "quality" of its art? If it's doing so because it evokes a favourable internal state whenever it observes it, could that not be considered art?
(When you listen to a piece of music and it makes you feel good, would you not consider that a good piece music?)