If they were going to show the full special, they'd probably need to schedule it to fill in an hour and a half or two hours and add enough commercials to fill out the slot. And that's what the Sci-Fi Channel used to do when it had Doctor Who and aired the Christmas specials; I don't know why BBC America is trying to force the 2009 specials into a single hour-long slot.
In my area, it's a specialty digital channel you only get if you get digital cable. I imagine that in other areas with more demand, BBC America might be a basic cable channel, but I don't know.
The more I think about it, the more it occurs to me that there's another reason American television tends to stick to beginning and ending programs on the hour and the half-hour: The nature of American network schedules and institutional inertia.
To be specific: The broadcast networks traditionally only got to program five or so of programming per night. They have to schedule everything around how the local affiliates want their schedules filled, and that lends itself to making sure that everything fits into standardized half-hour, hour, or, on weekends when airing films, two-hour slots. Cable channels don't have to appease local affiliates, but institutional inertia (combined with general social expectations -- seriously, no one in America likes it when you have to tune in at 18 minutes past the hour) probably explains why American television tries to fit everything into two-hour, hour, and half-hour slots.
