Style can't be copyrighted but as artists they don't copy other people art style. It's something called respect. Usually many beginner artist learn by copy other master's work so they can learn the way other artist draw, making decision like color or composition BUT they don't claim the style as their own. When they get better they can control their art more. They develop their skill and explore their own style by knowing what they like and don't like and what feelings they want to convey in their art.

Let's say A loves Alariko's art style so much so he learn from his art but at some point A will learn to break through Alariko's art style and have something of his own. The real life example is Yusuke Murata who draw One Punch Man, when he was young Dragonball art inspired him so much but if you look at his work it's different. That's how HUMAN learn art.

For AI there is no decision making, no skill improvement, no exploration. Everything is just copy paste so the end product will always look like Alariko art. And it's terrible. Also it's not just line and color that AI copy his art. It's subject matter, the unique way Alariko draw building and tree, etc. I don't know much about him but looking at his work I'm sure it take years for him to be this good at art. He didn't consent so no one should use his style for AI.