It has to do with who has the television rights. The B10 only has rights to the games played at their venues. In other words, television rights reside with the home team. B10 home games will be on ABC (with ESPN/2 coverage for regional games), ESPN, ESPN2, and BTN. B10 games may appear on the previously mentioned channels and: ESPNU, ESPN360.com, Versus, FSN, CSTV, TBS, NBC, ESPN+/GamePlan, and maybe others.
Here's a chat (search for "non-conference") that specifically mentions non-conference away games:
Big Ten Network - Mark Silverman Chat Transcript from May 24, 2007
As I mentioned, this probably isn't going to be much of a problem. The bigger problem will be regional prime time games.