The exchanges already have a "stake" on STEEM if they already have wallets here. So, they can reliably freeze some of it to allow themselves to vote themselves to the major witness positions. It is what they already do on EOS. However, STEEM is protected in part but the long 13 week powerdown, so they are vulnerable against large calls against their balance if it is locked up...

At the moment, quite a few exchanges are gaming around with the POS protocols... it is interesting for people don't know how to capture inflation for themselves, but a little worrying when it starts impacting upon the governance of a chain.