At least, that is what everyone thinks. Nobody can seem to find what the actual rule is. That being said, even if it were a 50% rule, the BTHC was formed when there was 6 of 12 programs. Not only that, but the conference begins play the year before Maryland and Rutgers come in, so the conference would be grandfathered in.
To answer TheMightyJobu's question, it either didn't matter (under the theoretical 6 team rule) or happened to late (under the theoretical 50% rule).
As for how this pertains to future B1G conferences (such as Lacrosse), nobody seems to really know, but everyone is under the assumption that the league bylaws require 6 teams. Again though, nobody seems to know what that actual bylaws say.