"Why is this not a 'just don't use the service if you don't like it' deal?"

Because that's the law in Europe (according to the complainer). It doesn't matter that you're free to not use the service, the law says that if you do use it you have certain rights.

You're free to not use any service anywhere in the world. But if you do use a service in some jurisdiction, that service is subject to the law in that jurisdiction.

Facebook, being Big Boys (TM), must follow the law of the land. If they do business in Europe, that law (it seems) will be more favorable to consumers than they're used to here. Tough.

Personally, I think they'll get away with it. Corporations are becoming their own law. Facebook may have made that calculation too.