Obviously this means that the miners could collude to steal everyone's segwit balances, but more likely, a government could force mining organizations to allow their transactions to steal bitcoins, reverse transactions, or whatever else. Whereas previously this meddling would've been extremely noisy, causing a hard fork (which would have likely been ignored). With segwit, however, theft by miner collusion is a first class feature.

Miners controlled from governments is the future we are facing.
The most powerful weapon we had for our freedom will be used for our slavery... Nice!