From what I have researched, its not about them being "ICO's" but instead its the fact the Bittrex is US based exchange, and these tokens could be considered securities. Of course yes, many being shit coins as well with broken wallets.
Bittrex has been doing a review since early January and delisting every few weeks. Either way its keeping them from any issues with SEC...
RE: Bittrex will remove loads of wallets March 30 - Withdrawal Now