(short answer)
The fine print states that text messaging is within the US, to US-based phones only. Anything else is charged.
(my rant)
Sprint seems to be neglecting SMS when it comes to roaming services. I can purchase a plan that allows for data roaming outside the USA, I can get discounted roaming rates for voice calls, but there seems to be nothing to cover international SMS. I hope this oversight gets taken care of soon, especially since I'm planning to travel to Canada in the next few months. I'm there once every year, and my Sprint phone always goes with me, but this is the first time I'm going there as a Simply Everything subscriber. My old plan didn't charge me for the Canada to USA messaging, but for some reason, I just don't trust that the new plan will be so generous.
Don't expect to get a straight answer from Customer Care when you call. They don't always seem to understand how similar the networks are between the two countries, and some often mislead you into thinking that you need a different phone to go there. You don't.