The whole "counted as P5 for scheduling purposes" was something that was thrown out in the aftermath of the first college football playoff to address perceived differences in strength of schedule. That notion initially met with a lot of raised eyebrows, and really it hasn't gained any traction going into this year. "Quality wins" and "quality losses" are the buzzwords of this year, and it doesn't look like conference affiliation (or lack thereof) weights very heavily into those determinations. If BYU wins, they are a quality win/loss regardless of being independent or in the AAC/MWC/"Best Of".
Big name programs get a bit of a pass on strength of schedule because their name gives them the benefit of the doubt. These big name programs really just want to have as many home games as possible, and BYU has been very helpful in providing these at a reasonable cost. That's what it comes down to.