As many people have pointed out, the singles line is not always faster. I have been in the singles line when there are many full groups in line and the singles line is actually slower.
I am pretty sure that that singles lines were created to fill chairs with people so the lines move. Long waits = unhappy customers. I would bet that ski areas could care less if those people are part of a group of skiers or not.
Single skiers in the group line NOT grouping up and taking a whole chair to themselves is what slows the line more than anything. Right that resorts don't care as long as people in the singles line don't try to regroup before unloading. The more chairs at capacity the better for all though. Singles not paring up = lots more empty seats than when a singles line encourage them to double/triple/quad/six up.