I think the fundamental difference is that you need a player at every source of xp/gold. If you choose to top or jungle or mid then don't, your leaving one of those income sources empty, and not only that, uncontested by the enemy. You wouldn't be able to maintain a 50% win rate by doing so.

On the other hand, the "support" singed is merely choosing not to support the ADC, but to immediately the support the rest of team. All he is doing is shifting shifting the pressure the support normally applies. That pressure still exists. Additionally he isn't (during the time a conventional support would be bot lane) touching an existing income source, but rather the enemies.

Is his ADC now under more pressure because of this? Absolutely. But so are other players on the enemy team. There is nothing in his actions that inherently makes the teams unequal, the way having an empty lane would.

He wins more than he loses, so clearly something he is doing is working.

He is guaranteed to be creating a negative experience for one player (his team's bot) and very likely to be creating a negative experience for the other 3 players.

Any thing unconventional is guaranteed to do this. Not because unconventional is bad, but because people don't know how to adjust and play with it. They get worked up over an action they have no control over. The only way to fix that is for this action to become not so unconventional.

That said, if riot truly feels that this isn't the correct way to play the game, I don't think banning him is the right action. As long as a player is honestly working to win he or she shouldn't be punished by how he chooses to play the game. If riot feels he is playing wrong, the solution isn't to ban him, but to design the game so that it doesn't support his play style.

Finally:

Even if he wins slightly more then he loses he is doing so at the experience of the experience of a lot of other people.

If he wins more than he loses then his teammates are only getting upset due to their ignorance. That is not his fault.

Furthermore, if he loses with this tactic it can most likely be directly attributed to his actions.

This is nonsense. If there has anything I've learned from getting all the way diamond is that its pointless to place blame on other players. The only games where a single player is directly responsible for the win or loss is when there is a massive skill differential or intentional feeding. I'm not just speaking to this situation, but league in general. For example I was just in a game where the enemy mid was 6-0 very early own. Yet he still lost. He blamed his team, saying that if they weren't so heavy he could have won. But the truth is also that if he were a better player he could have won. As the player with the most kills early on he could have directed the flow of the game but he failed to capitalize on his advantage, so all that gold really didn't have the effect it could have. Except for games that aren't at all close, it is futile to attempt to "directly attribute" the blame to one person. League is a team game and every player has the ability to control the game. If you fail to do so, it doesn't mean the fault lies solely with others, it means you just as them failed.