I don't think it's that. For example, when I'm roaming, my carrier charges me 40p/SMS and 20p/MMS. You can perfectly easily send text messages by the latter. I think they're just trying to encourage people off the (presumably) older and creakier SMS infrastructure onto something fitter for purpose. Don't forget the SMS started off as a useful tool for engineers to test things; the carriers were taken by surprise when it took off with customers, so I bet the infrastructure they rushed into production to handle it is a mess.
There's no reason why IM and microblogging can't supplant SMS. Even ordinary phones are enough to run clients for those nowadays. The fact that it doesn't indicates the market is broken.
