This post may get moved to the proper section, but I'll give you my $.02 anyways.
Usually it's to have everything well balanced through the PA. I'm not a sound guy but as far as drummers go, there are few people that can make you blink with a backbeat the way I do. So when I'm playing rock, I have had many soundmen look at me with raised eyebrow.
I feel that our situations are the same. The reason is, if I play quieter then I am holding back energy and not getting the same tone that I'd normally get at full volume, similar to your guitar amp having a sweeter tome at higher volumes.
The problem is, the venue and PA may not support those kinds of volumes while having the total sound to the audience still be balanced and in control. So it ends up being a compromise.
At the Flaming Bird for example there are no live drums. There's a reason for that. I was killing the room...lol
I usually always give the sound guy the benefit of the doubt in these cases, within reason of course, I use my judgement because I've also dealt with many sound guys that were just retarded.
One time a guy didn't like the sound he was getting out of my Ludwig kick drum (a drum that I was using successfully 4 times a week playing shows at the time), so when I came back he did me a favor he said by cutting a mic hole in the front head. Needless to say I now know what it's like to contemplate murder.
Also I then realized the sound guy isn't always right, so I guage the situations case by case. But usually the sound guys around here are pretty good.
Someone else can give you the "scientific" reasons, what do I know, I'm no musician, I'm just the drummer.