Had a similar sitch in a BV game just a week ago. Partner was administering throw-in along the sideline, where A1 decides to throw the ball off of B1's back and pick up the loose ball and cut towards the basket. I moved towards the basket as L to cover the play when I hear a whistle. Partner blew the play dead and came to me (I was R) and said that she felt it was unsportsmanlike conduct because B1 was on the 3-point line, and A1 had to throw it a little hard to make that distance. I told her lets go with warning, give the ball back to A1 for a throw-in, and move on. I sent her off to explain to the coaches, while I went and explained to A1. A1 was about to give me a dirty look, and I told him while I didn't think it was a good call, his actions still was suspect and told him to go easy on it next time. A's coach was not too happy with the explanation but accepted it, while B's coach was like whatever.