The miners. This is why bcash is a bad idea, and why drivechains aren't an alternative to softforks.
Discussion
Miners do not have custody.
Miners are profit seeking. If they start censoring sidechain transactions then guess what they lose out on business. And will be outcompeted by a btc miner who doesn’t censor sidechain transactions.
A Drivechain steal requires a coalition of miners to control 75% of the hashrate for 6 months.