Utah and TCU left for something better. Something that is OBVIOUSLY better. Something that each and every school we're talking about understands and would have done if they had that option.
BYU didn't leave to go to something better. BYU left because having NO conference was better than having the Mountain West Conference.
You get it if your girlfriend dumps you for the rich and handsome millionaire doctor who everyone likes and looks up to. But if your girlfriend dumps you simply because you aren't good enough - and she doesn't have another dude she's planning to date, she just knows she'd rather be alone than be with you - that's the kind of stuff that makes a dude bitter.
If you don't see the difference between what BYU did and what Utah and TCU did, then I don't know what to say, other than it is very obviously different circumstances.
That said - I was onboard with BYU leaving when they did. I just understand why those that were left behind in the MWC were bitter about it.