TL;DR
In May, Kid Cudi removed M.I.A. from his Rebel Ragers tour after a Dallas set where she told a booing crowd she'd been canceled for being a "brown Republican voter," called for a free Palestine, and went at ICE. She sued on May 29 for $2.8 million in guaranteed fees plus about $290,000 in lost merch, VIP and a private booking. On September 8, Cudi's lawyer Mathew Rosengart filed to dismiss, arguing the suit "fails on its face" because a performer choosing who appears on his stage is protected by the First Amendment.
The two arguments
Her side: a contract is a contract. She was booked, she performed, she was fired for what she said, and she says Cudi used it as a publicity stunt for a tour that was underselling.
His side: it's an anti-SLAPP-style argument. Deciding who shares your stage is expressive conduct. Forcing a headliner to keep an opener whose message he doesn't want attached to his show would be the government compelling speech.
Why it's interesting
Both sides are claiming free speech. She says she was punished for hers. He says the punishment was his. Courts have generally been careful about letting the First Amendment erase a signed contract, so the motion isn't a lock — but the argument that a live show is itself a piece of speech has real footing.
Keep it in proportion: this is a business dispute between two artists, not a movement. It'll be decided on paper by a federal judge in California.