That's a generous approach and undoubtedly it was very close to the line, but everything I have seen strongly points to the fans reaching out on to the playing field side of the fence at the point of interference. That said, I am inclined to think that at some point in the transaction, Mookie "broke the plane" reaching over the fence the other way too. But as I read the rule, even if so, if a fan reaches over the plane on to the playing field side and plainly prevents the catch while doing so, that is fan interference even if the player is at more or less the same time reaching over the fence in the other direction. In other words, if both the player and the fan are on the wrong side, and the fan plainly prevents the catch, then it is fan interference.
One thing that I wish the media would tumble to is where the actual plane is. If you look at the right field wall, the entire top of the wall is painted yellow which means that a ball that hits the top of the wall is in play. Contrast this to the Moster, where the shelf above the painted red line is not in play and is a home run. That means that for purposes of defining what constitutes the "playing field side of such fence, railing or rope" within the meaning of the rule's approved comment, the playing field side is the part of the fence that lies outside the plane that extends vertically upward from the inside (that is the part facing the spectators) part of the wall.
When this is understood, I really don't think the call is that debatable in the absence of a more dispositive angle. All the video evidence I've seen suggests it is more likely than not that Mookie's glove at the time of interference was on his side of that plane.
And one final point. The relevant question is where the fan's interfering body part is at the time of the interference, which may or may not be the same as where his body part is at the time he touches the ball. Interference is the act of preventing the catch, which started to occur here with contact with Mookie's glove not with contact with the ball.
Again, a view down the wall would be crucial. Without one, the evidence that exists to me suggests it's more probable than not that Joe West got this one right.
There is another part of the rule that is not really being talked about. The interference must prevent the catch, which means the umpire has to judge that the player was going to catch the ball. I think it's fair to say Mookie might have caught the ball. I'm kind of curious how umpires are taught to judge this. Do you presume the ability to make a spectacular play? Can you take the attributes of the player in question into account? Or do you assume something closer to ordinary effort? Is the rule that a ball that has the trajectory to be a home run should be presumptively judged likely to have made it or do you decide that you should never punish the player from being deprived of the opportunity to make a great player if the spectator violates the stadium rules? I think these are all pretty interesting questions and not obvious.