I wouldn't be surprised if other carriers do that too.
Sprint was charging $0.15 per message, and then raised it to $0.20. That of course included every message sent or received. Since my friends were only increasing the volume of messages---despite my cease and desist notices---I opted out and now receive no text messages at all.
Incidentally, most of my friends don't think much of my argument that phones should be used for voice communication, and that only email and IM (and snail mail, I suppose, though who uses that anymore?) should be used for text communication.