is it bannable?

Is playing ranked with a smurf bannable. The smurf was unpaid and only doing it to help his twitch viewers trying to get his stream popular and practice new champions. I was in a dynamic queue of 5 and was the only one to get banned. Riot Tantram told me I was banned for paying a booster. The word "paying" by itself indicates that playing with a smurf is allowed as long as you don't pay them. Riot refuses to check my in-game settings, frequently play champions, previous elo, or my IP address to determine if someone else was on my account (no one else has access). The account started this season at S4 and got banned at S1 2/1 in promos. Previous elo is gold 5. My main was also gold 5. To prove a point, i am playing the same champion to gold while only playing solo queue (no duos). I am being lazy on leveling another smurf but it is currently in S1 promos gaining 25-26lp per win.

So the question again, is playing with a smurf who is NOT being paid, a bannable offense. Also why was I the only one in a dynamic queue of 5 to be target banned. My account had 0 offenses including toxicity as far as i can remember and riot won't tell me what i did to get banned besides paying a booster.