Keeping your hands to yourself is something we learn early in life. And generally speaking, people are pretty good at it. But not for not wanting to touch; there are, after all, many smooth and soft and bumpy things out there, just begging to be touched.
But, typically, there are penalties for touching what's not yours to touch. Upon seeing an attractive woman on the street, you can't grab her just because you feel like it, and you can't walk into a museum and run your hands over the art just because you feel like it.
If you did, the penalty would be severe, and rightfully so. As adults, we have to control what we want (or better yet, we should learn not to want certain things; you may want to poop in your pants when you're two, but hopefully not anymore.) And who do you think you are, anyway, diminishing that woman's dignity, or ruining that painting so that countless others cannot enjoy it?
And yes, grabbing a ball in play is definitely the same kind of thing. You shouldn't want to do it at all, and it drives me nuts when it does happen, to the point of advocating draconian (if not Franconian) punishment.
Consider the following. When a fan reaches into the playing field, he's implicitly saying a number of things, each entirely indefensible.
First, he's saying 'I hereby claim ownership of this ball, this ball which I didn't make, buy, or even ever see before two seconds ago, and which nobody offered to give me.'
(A rejection of the Lockean principle that ownership derives from "mixing one's labor" with an object.)
Second, he's saying 'even though the fate of this ball is the intense emotional concern of millions of people, I am going to take it so that only I can enjoy it.'
(A rejection of the utilitarian principle of the greatest good for the greatest number.)
Third, he is saying 'I am claiming this ball as mine at the only moment of its existence that's ever mattered, at that very moment at which all its previous existing moments were aimed.'
(A rejection of natural law ethics, which holds the interruption of teleology to be immoral.)
I think saying these things makes you an asshole.
And there's no stealing a loaf of bread to feed the starving family mitigating factor here. Unless it's a milestone homer, when removed from its context as a ball in play, all it is is a dirty ball. And clean balls aren't expensive. By taking it from the field, just like smudging a painting in a museum, you're making it worth even less.
And for what? So you can look like a jerk on TV, publicly declaring your selfishness? So you can regale your grandkids about the day you interrupted what millions of people were doing for no good reason other than that you felt grabby?
In short, you reach into the field of play to grab a ball, you're saying that having that particular small round leathery thing is more important than the happiness of millions of people.
I think it's pretty obvious what we should do with people like that.