We don't know if he never bet against his team. We admittedly only have proof that he bet for his teams to win, but given that he denied even that for a couple decades I don't take his claims that he never bet against the Reds at face value. Once a liar, after all.
And even if he never bet against the Reds, the fact that he bet sometimes obviously raises into question if he managed games differently. If he didn't have money on a game, did he rest players to have them ready for a game he did bet on (specifically if he ever didn't use a cluch closer or reliever where a circumstance otherwise merited it)? We have no way of knowing, and that's why a blanket ban is required, as the act of betting on some games invariably and logically impacts the decisions you make on the games you don't bet on. Or did he burn through players to win games he bet on at the cost of future games?
Not to mention that I have a hard time imagining that him not betting on some games or betting on other games didn't impact bookie behavior and oddsmaking, and so who knows if he gamed that to any degree.
Are there bigger dirtbags than him? Sure. But gambling on baseball has been a universally banned thing for personnel involved in the game for generations. It's basically the one red line that MLB doesn't walk back from, and Rose is so damn high profile that they have to stick to their guns to show that if even he can get banned then so would random journeymen and minor leaguers if they get caught, too.