I agree that every diver should check his/her own air....but a lot of them don't. The ocean bottom here on Guam would be COVERED with dead divers if the guides didn't check their customers' air. Other guides who work in areas where many of the divers are infrequent, once or twice per year divers know what I'm talking about. I know some people don't want anyone to touch their gear...if that was the case with my customer, fine...except I'd need to personally watch them check their air. Normally a lot simpler just to check it myself. I have 6,270 dives as of yesterday, and most of the time when I'm about to do my entry, the deckhand will check my air. I already KNOW it's on, but it's not hurting anything and lets him be another "link" in the chain to increase diver safety, so I never ask them not to do that.
As guides, the customer's safety is the most important consideration, and some people need closer attention (sometimes LITERALLY...and I know the difference between literally & figuratively...having their hands held throughout the entire dive, because they are incapable of effectively moving through the water themselves.) For people who are really opposed to things like having their air checked, it is, of course, possible to dive unguided or solo, in which case your safety is your own concern, and there's no DM/guide who has to explain "I didn't check that person's air because they didn't want me to." Again, the guide doesn't have to "touch" the customers' gear, but one way or another must be SURE that the air is on.