This seems like one of those cases where you need to assign responsibilities and obligations to those enabling the damage, even if their offerings also enable a lot of good. If you have the capacity to offer cheap/free VPS, then you also need to cover the cost of protecting against the DDoS attacks that service enables. You don't get to offload that burden on to the victims. If that makes your VPS offerings more expensive then so be it; that's the result of pricing in the externalities.