There was a large bit of media last year about cost caps and limitations. But, hey, that only covers part of your bill since Texting is a different "service". This is how Bell gets away with it. They may finally back off now that this has hit the national/international news but it shouldn't need to reach that point.
How is the teenager supposed to know her text is going to the US or is being sent from the US? Maybe she knows where her friends are and maybe she doesn't. In any case this can happen to absolutely anybody at any time. I don't know where my friends may be located at every moment of the day (I guess they take vacations sometimes). And with the constant barrage of spam I doubt I would even believe a random message telling me to pay up.