No, it isn't a bug. Unfortunately, telephone regulations allow some local carriers to get away with some unfriendly charges to the big phone carriers. Google made a choice to block those rural (not 900) numbers because they currently aren't as regulated as a "real" phone company.

I hope that this will change the rules for carrier charges, but I also think Google needs to play by the same rules as everyone else. Blocking rural calls is evil when the rural users have no knowledge of how the system works and are just innocent victims.