Sometimes, you just have to wonder what is going on in their minds as they reach over the fence to snare a ball that's in play. Is the urge to grab that souvenir so great that they could be willing to interfere with the game going on?
So what happens when a fan interferes with the ball? Well, the game must carry on, so to speed things up, the umpires just decide what "would" have happened if the fan did not interfere. People often confuse fan interference with a ground rule double because usually when a fan interferes with a ball in play, the hitter is awarded a double, and the runners advance two bases. No pun intended, but you should realize how faulty that system is right off the bat. There is no way an umpire can know what actually might have happened if the play wasn't interfered with. Though they are major leaguers, the players are still human and are highly capable of making an error or two.
A recent example: Friday's exhibition game against the Rangers. In the bottom of the first, Rich Aurilia was batting with Jose Cruz, Jr. on first. Aurilia battled in an 11-pitch at bat. Finally, on a 3-2 count with Cruz running, Aurilia lined one ticketed for the left-field wall. However, a fan reached over the wall and caught the liner. It was a very nice play, but it was still interference. Aurilia was credited with a double, and Cruz was held up at third. Cruz had been running on the play and would have easily scored if the fan hadn't reached in and caught the ball. Simply put, the interference cost the Giants a run. Luckily, it was only an exhibition game and not a regular season game. Problem is, this happens in regular season games too. Plays like this can quickly change the course of the game. What's not to say the ball is misplayed, and Aurilia advances to third? Or even goes all the way home? Ah, but we'll never know… because of fan interference.
It's actually quite straightforward folks. Let the players play the game. Foul? Dead? Go for it. Live? Leave it alone.