If you're looking for a universally-applicable moral framework, join the thousands of years of philosophers striving for the same.

If you're just looking for an explanation that allows you to put one foot in front of the other...

Laws exist for us to spell out the kind of society we'd like to live in. Generally, we prefer that individuals be able to participate in cultural conversations and offer their own viewpoint. And generally, we prefer that groups of people don't accumulate massive amounts of power over other groups of people.

Dedicating your life to copying another artist's style is participating in a cultural conversation, and you won't be able to help yourself from infusing your own lived experience into your work of copying the artist. If only by the details that you focus on getting exactly right, the slight mistakes that repeat themselves or morph over the course of your career, the pieces you prioritize replicating over and over again. It says something about who you are, and that's worth appreciating.

Now, if you're trying to pass those off as originals and not your own tributes, then you're deceiving people and that's a problem because you're damaging the cultural conversation by lying about the elements you're putting into it. Even so, sometimes that's an interesting artistic enterprise in itself. Such as when artists pretend to be someone else. Warhol was a fan of this. His whole career revolved around messing with concepts of authenticity in art.

As for power, you don't gain that much leverage over another artist by simply copying their work. And if you riff on it to upstage them, you're just inviting them to do the same to you in turn.

But if you can do that mechanically, quickly, so that any creative twist they put out there to undermine your attempts to upstage them, you have an instant response at little cost to yourself, now you're in a position of great power. The more the original artist produces, the stronger your advantage over them becomes. The more they try, the harder it is for them to win.

We don't generally like when someone has accumulated tons of power, especially when they subsequently use that power to prevent others from being able to compete.

Edit: I'd also caution against trying to make an objective test for whether a particular act of copying is "okay". This invites two things:

1. Artists can't help but question what's acceptable and play around with it. They will deliberately transgress in order to make a point, and you'll be forced to admit that your objective test is worthless.

2. Tech companies are relentlessly horny for this kind of objective legal framework, because they want to be able to algorithmically approach the line and fill its border to fractal levels of granularity without technically crossing the line. RealPage, DoorDash, Uber, Amazon, OpenAI all want "illegal" to be as precisely and quantitatively defined as possible, so that they can optimize for "barely legal".