This is not really surprising. Even in the case of DAO, the SEC had ruled that the tokens constituted a type of security. However they decided not to press any charges. The ruling was intended more to be a guidance for future ICOs. Similarly, in this case they are letting bygones be bygones.
It would be futile for SEC to pursue the whole thing. They better look at the situation with more understanding on a broader perspective including formulating a better win-win formula for both sides. They have to accept that cryptocurrency is here to stay and that in the process the government can also be the winner instead of just being a bitter antagonist. We are hoping that this acquisition can signal a better era for cryptocurrency in USA. Under the management of Circle, Poloniex should experience more growth and can finally offer better services rivaling the top exchanges on the market right now.