tptacek parent
I would take it down, not because of ethics or legalisms, but because you'll lose technically. They're making it clear that they don't want interoperable implementations. All you're doing is poking them in the eye with a stick. You probably don't have the resources (especially given your lack of interest) to keep your implementation working; they certainly have the resources to break your implementation. Why bother?
Snapchat probably does not want to break all of their deployed-and-delivered apps.
If they shipped the app/service without a "force the user to update" feature, they would risk leaving thousands of users in the dark.
And if they did modify their protocol, it would probably get reverse-engineered again, either by the OP or somebody else.