I don't know if the OP of this reddit thread is correct, but he says there are rules preventing Bitfinex withdrawing coins from BitGo wallets belonging to its customers until an arbitrator decides what to do.
Bitfinex emptied all BitGo wallets anyway, but whether that was legal is an interesting question.
https://www.reddit.com/r/Bitcoin/comments/4wcspe/i_recommend_to_those_from_bitfinex_with_bitcoin/I recommend to those from Bitfinex with bitcoin left in their wallet to login to BitGo and freeze the wallet. According to TOS 5.5 BitGo/Bitfinex is obligated to no longer use their key to move funds until an arbitrator has ruled on the dispute