So by your logic, if someone sets up bots at a computer shop, say about 40 of them, and about 5 of you guys are "legitimate" I should bot check all 40 of those accounts, and if 3 of them are not botting, including your 5, I should not ban those 8, but ban the rest?
What if you are gaining money from the botter? What if you were gaining items from the botter? It's just as illegit as botting yourself. The rules are there, and it tells everyone what the consequences of botting are. If you were not botting, you have no one else to blame but the botter. He knew he'd get in trouble, and he knew that he put you at risk too. He just didn't care until he got caught.