[mimentum]

SMS runs through a different protocol and has different set standards; https://en.wikipedia.org/wiki/SMS

iMessage is a proprietary messaging system

[wintermutestwin]

Any idea why so many providers charge for SMS?

[kelnos]

Simply put, because they can. They may have a monopoly or near-monopoly on SMS service, so they can do what they want.

That was the state of affairs in the US until fairly recently; most carriers (in the US at least) offer a pretty standard plan that includes unlimited texting.