When two neighboring countries are on the same continent, like the U.S. and Canada, the citizens generally do a lot of traveling back and forth between the countries. Many have relatives, friends or even conducting business that requires travel. Before the day of cellphones, it was no big deal - you drive or fly to your destination and when it came to communicating, you picked up a landline and made your long distance of collect call. The only thing that 'roamed' back then were the buffalo on the western plains.
With the advancement of the smartphone, many landlines are disappearing - we haven't had a landline in years...like we need another $40 a month bill. We take our cellphone with us everywhere and use it as our only means of communication. However, cellphones need towers for our signals and if you stray too far from your home area, there is a nasty roaming charge attached to your next bill. Roaming rates are generally very high when compared to the normal rates - the Quebec City Mayor Regis Labeaume just recently ran up a $20,000 bill due to roaming charges while in 'Rome' (yes, I see the irony) and another $2,106 bill using his tablet in the U.S.
The carriers try to sign agreements with other carriers to allow their subscribers to 'roam' on the other network - they try to work out a deal to reduce costs. Even the Canadian Government has passed new laws that limit the amount that carriers within Canada can charge each other. However, once you leave the country, those laws no longer apply. AT&T in the U.S. seems to be on the forefront of setting up agreements with our Canadian brethren - last year at this time they firmed up and agreement with Rogers and just recently, they have struck a deal with Bell.
If you go to Bell's updated U.S. roaming rates, on Band 4 and Band 17 LTE, AT&T service can now be accessed...meaning that both AWS and 700Mhz supported devices will be able to find a signal. A partnering agreement with T-Mobile is offering AWS-based 3G as well. The prices are not the lowest by any means, but come close to TELUS'. Rogers' "Roam Like Home" still seems to be the best deal. It is also good to know that Bell now has the better speed than TELUS.