I think the answer is simpler. There are so many AA meetings that rehabs can easily find one that fits their schedule to send its people too--and can be sure that the program will be available to the client when he or she returns home. The other programs have fewer meetings and might conflict with the rehab's schedule of activities and do not exist in every community.
I think in my rehab if I had found another program and wanted to attend its meetings no one would have stopped me. As long as a program does not contradict what goes on in rehab, would any rehab really care?