Ever since the SMS service was launched back then in the mid 1990's, most users around the world only pay for SMS messages they are originating but not for SMS messages they receive. And that is a good thing as users don't have control over incoming SMS messages and whether they want to receive them or not. This has led to the interesting situation that when roaming to other countries, incoming SMS messages are still free, pretty much the only roaming service in GSM that is free.
I am sure some network operators over the years would have really liked to charge for incoming SMS messages to users while they are roaming in another country but perhaps they decided it was not worth the effort of then also providing users with the means to deactivate SMS reception while abroad. Incoming MMS messages are charged when roaming by the way but most if not all phones today have an option not to download incoming MMS messages while roaming.
While this is pretty much the status-quo around the world, North American carriers are a bit different as some (if not all?) have started charging for incoming SMS messages some years ago. In other words if you decide not to take an unlimited texting plan and you get spamed, you pay for it. And it seems that happens more and more often as suggested by this post over at Gigaom. In a part of the world where companies are sued for much less I wonder why that hasn't happened so far!?