Why why why? Players often question a DM, when they think they are being cheated out of something. I noticed this happens a lot more in the new editions then in the older editions. Why? because the later editions have so many rules, and so much written out of things you can and can't do. Basically they took the adjudication away from the DM.
Why did they do this? Well, I think its because people expected more with new rules... stressing on the rules part.
I guess what I am thinking or trying to say, is that I like less rules, because I can decide what works, and 99% of the time my players dont mind because it keeps the pace of the fun moving along.