I think it mainly comes from how uncomfortable it is for the OB/midwife/nurses to hear someone making that kind of noise (and truthfully, I see this most with OBs, at least at my hospital). It's not a societal norm we have, and so people are asked to stop and coached to do the quiet, closed-glottis thing. Even in cases where open-glottis pushing takes longer (and I do think it tends to), I think it's how uncomfortable it makes the care providers, rather than the time, that drives a lot of (over) coached pushing.
Personally, I think if you, as a care-provider, have a problem with how someone looks in natural labor, then it's *your* problem, and the onus is not on the patient to conform to something you like better. If there's a mom who is screaming during a push, or not pushing effectively, I'll often tell them to try "grunting against the pressure", or making the same low-pitch sounds I encourage them to use during labor. In my experience, it works well, particularly with moms who are going sans-epidural, or have some feeling. If a mom is just totally numb (can't feel her contractions, has never had a baby, needs lots of coaching for pushing), I'll labor her down if I can (per OB), and include open-glottis in my coaching. Sometimes the OB will come in for delivery and tell them to be quieter (in so many words), but by that point, the baby's almost out. Open-glottis pushing isn't something that's caught on much in my hospital, but I use it in my practice, and so do 2 of my co-workers.