It would be pretty much impossible to communicate this information in advance.

There are hundreds of mobile providers in the US, some of which only cover very small RSAs (rural service areas). For the cellular companies to try to communicate to customers the full list of all service areas, and whether there are roaming agreements in place would be nigh to impossible, especially as (a) the owners of the licenses is in a constant state of flux, and (b) if signal strength from one provider is weak, you may pick up another with which Rogers etc. do not have a roaming agreement, and thus get the "thank you for calling cellular express" (etc.) message and the credit card prompt.

Simon