Yes, your contract is with at&t, and in there I am sure there is something about roaming. at&t has a contract with aeroplan to provide roaming service to its customers. You did decide to roam by leaving your phone on the way you did, no different than boarding a cruise ship.

at&t just decided to let it slide because they figure future business is worth not angering a customer for such a small amount. Plus, I am sure the rates they charge are much more than are actually paid, so they are not out the $100.