Shiv,
PGA always more player friendly set up - more like tour event and these guys are closer to them as a players organization. Also, don't want to embarrass club pros in field.
Medinah greens have stressed in previous tourneys. More water may equal more safety from Royal Liverpool look.
PGA doesn't want a Shinney or Olympic moment. They may not have wanted a British Open tournament with everyone complaining about brown grass.
I guess I wonder if other than the trees there was something going on. Hard to believe its agronomics, since more water would just foster more disease in the heat. However, when I worked summers at Glencoe to put my self through college, one night I failed to water, "knowing" the rain was supposed to come. It didn't and the greens stressed. The rule is if you need to water, water until it starts raining, and sometimes even in the rain - if you need 0.25" and stop in the rain, but it only rains 0.05" you can be just as screwed as if you don't water at all.
However, Medinah probably has a few more experienced guys out there plus a weather station that might have shut the water off had it started raining after irrigation started. I agree it shouldn't have happened unless appearance was more important than challenging the top players to the PGA.
And, it may very well have been. We are still a small subset of their audience. Most couldn't care, and may prefer 18 under winning scores.