Why would they disqualify him? He gained no advantage by stepping on the line and this was a semi. You don't dq your best sprinter in the semis. They've been pretty lenient all weekend anyway. Rollins wouldn't have made it past the first round otherwise.
Seriously, he gained no advantage? He ran a shorter distance than the other runners. Maybe he should have moved into the next lane completely. Would that have given him an advantage?