As previously explained multiple times, it doesn't happen often [anymore] because the TV networks stopped showing them. In the most recent ones you see as soon as the fan gets onto the field/rink/court the camera immediately switches to a view where you can't see the person. They realized that the more attention they give them, the more reason people have to do it. Who really wants to go to jail if you're only seen for a few seconds by the crowd, rather than a being replayed on sportscentre for a day or two and making the morning news?
However I can understand if one thinks it's just the guys in the truck asleep at the wheel again, as they do seem to completely miss technical fouls and scuffles, and botch breakaway dunks by switching views at the last second.