Some event promoters get really really pissed if you get in without consent to do photography. They try to control their media and what they put out to the public. Some "Dude" just shows up and plasters crap all over the internet can get attention. I was at an event that some "Dude" was at. The promoter ended up threatening to sue him. Nothing came of it besides the guy having to pull some images off the internet. But not before I am sure he sold a few images.
On the flip side if "you" are an event photographer for "money" and some "Dude" shows up when they are not supposed to be there you get really pissed. Because you are there for money. Especially if you are the exclusive photographer. And if you are not selling shots because the dude is giving them away it takes money right out of your pocket.