I don't quite understand why feeding parking meters is illegal.

Bear with me...

In some cases of abuse, like with NYC metro cards, there is a legitimate reason not to use a single metro card for a number of people (the limit is four I believe). The reason is because putting a large enough money on the metro card to accomodate those people automatically gives extra money as an incentive for large purchases.

But if you use this other people, you're technically ripping off the system, because those individual $2.50 cards wouldn't have gotten any extra percentage.

So in examples like the foregoing, I get why it's illegal to "share" money. But feeding parking meters has no such incentive anywhere, as far as I know. Why would it be illegal?