Because we've (UAL & CAL) always "bought" the RJs one way or the other. Either we bought the aircraft outright and leased it back to the regional, or we leased the aircraft ourselves and then sub-leased it, or we "co-signed" the lease agreements.

The regionals don't have the capital to buy RJs without a deal from the mothership. Nobody in the in their right mind will loan $$$ to a regional without a mainline deal in place.