While smurfing just to win is bad I personally admit to smurfing when trying to teach a new player or a friend the game. The main reason I do it on another account is that it would be less fair having my runes and mysteries at a low level with a friend and it's hard to teach them about things like warding and the effects in a bot game when trying to go into detail bot games can only teach so much.
Plus when someone is new they enjoy playing with their friends obliviously I don't look at smurfing as a whole to be a bad thing but doing so to just screw with other players is.
There are many reasons players smurf aside the reason I stated previously, I mostly just bring friends into arams as it's the least toxic mode and the mode I personally have the most fun in. But they don't really unlock that until level 6.
I have known people who have made new accounts simply to learn a new roll such as creating and account on only going support or jungle/etc. to learn a roll without getting snowballed by players at their own level and learning nothing.
There are many ways to look at smurfing but I can tell you there is a large amount of players that do it for no other reason but to feel good about beating lower level players or to just piss people off on another account.
I'm not sure what you would do as far as punishment for this because you would also have to punish those who do so for none toxic reasons. If they player isn't in chat being a jerk etc. It's rather hard to tell why they are doing it in the first place.
I will end on this note while not in league specifically I have tried playing other games and had a hard time learning because other players would make new accounts just to make it harder for newer players on purpose.