It's both. League of Legends itself is a game, but of course it is owned by a business: Riot Games. As a business, Riot needs to uphold their policies to make sure that the gaming experience is nice and fair for everyone, or at least as much as it can be. Now, as to whether or not they will punish you for account sharing, who knows, but you should know that coming clean and being honest about breaking one of Riot's rules does not automatically make you immune to any type of punishment.
As for you going back to bronze, Riot is not going to do that for you. If they did, then they would have to do so for anyone else that asked, and then you would have people moving up and down the ranked ladder without winning/losing the proper amount of games, which completely destroys the integrity of ranked. If you want to go back to bronze from gold, the only way to do so is to lose a bunch of games.