What's an artist who depends on the platform to do?
Labels can only write up contracts giving them rights because they pay for studio time (given them a financial buy-in to the creation of the music) and pay advances against royalties. Not so for indie distributors. I don't know if there are specific applicable laws, but it's absurd enough that everyone would immediately drop Bandcamp.
Music royalty rates, for physical records, radio and internet streaming, are also determined by a panel of judges: the United States Copyright Royalty Board.
I guess that only works for Darth Vader.
A small team can quickly copy how it works and get all the artists to move platform if they were to shoot themself in the head. Right now this doesn't happen because Bandcamp functions and exists and has a reputation for musicians.
Lose that, all the artists start looking for a new home and someone will make an alternative quickly.