They aren't giving up two spots.
As an independent school BYU's path will be very limited.
BYU won't win the AAC every year but I believe they can compete for their division most years which gets them in a conference championship with a shot at the playoff. As a member of the AAC BYU will have a path to the playoff most years on November 1st. As an independent that path will be shot by mid-September. Playing Tulsa in November with a potential playoff spot on the line is infinitely better than playing at USC in November with nothing on the line.