Well, I'm guessing that the new T&Cs are measures to improve response time.
Currently if you go by the no-reservation route, it takes a minimum of 4 steps -
1. submit BRG
2. have it processed and receive tentative approval by email
3. make reservation and reply to email agreeing to BRG terms
4. have it processed and receive confirmation
Maybe there are a significant number of submissions which end at #2 ie the guest never replies to the email for whatever reason.
If you go by the reservation-first route:
1. Make reservation and submit BRG
2. Have it processed and receive confirmation
So it immediately eliminates 1 processing step for the BRG team, and also hopefully cuts down on non-responders.