Unlike the rest of the world, SMS users in USA and Canada are charged to 'receive' SMS. It may seem bizare but it is true. Presumably it was this daft business model that Twitter hoped to persuade the rest of the mobile world to adopt.
As well as depressing usage, the 'receiver pays' model is one that rewards spammers and one that is not going to be adopted by other mobile carriers/operators.
Sitting in their Californian bubble this seems to have escaped Twitter.