I can say for me, it had nothing at all to do with me not being able to fix him, I never thought I could. It was as babyblue says, the feeling of abandonment. You stress and fear and walk on eggshells - then they go off to some expensive spa for misfits ($26,000 worth) while you are left to wipe the remnants of poo off the counter top and get on with LIFE.
While they get cheered on (right on! Good on you getting HELP!!! yay!) we (the families) got told we were stupid codie enablers and we allowed ourselves to be used and abused.
That we were loser magnets. Yes, they actually said that.
Anyway - for me it didn't have squat to do with some sense of indignation that I couldn't fix him and everything to do with what I perceived to be rewarding bad behavior.
For what it's worth.