Copyright law on this varies, actually! In the UK, "fair dealing" actually has an exclusion for using copyrighted material for the purpose of commercially competing with the creator. This also includes derivative works. This does therefore cover style to a certain extent, because works imitating a style of an artist are generally intended to commercially compete with them. From that perspective, taking an artist's entire portfolio, feeding it into an AI, and producing work in their style at a lower price than the artist does (because an AI produces something in seconds which takes the artist weeks), is pretty obviously an attempt to compete with the artist commercially.

While people like to draw comparisons between AIs and humans copying another artist's style, the big difference here is that a human artist needs to spend hundreds of hours learning to imitate another artist's style, at the expense of developing their own style, while the original artist is also continually developing their style. It is bloody hard to imitate another human's art style. But an AI can do it in minutes, and I haven't yet seen any valid arguments for how that's not intended to commercially compete with human artists on a massive scale.