The contact from behind wasn't that bad, but when #15 pushes the other player in the back of the head with his left hand it became a UF (C2). You can tell from the hand gestures of the official while explaining it to #15 that this is the reason for the call.

exactly its not a legal defense hence the foul (already enough punishment). UF was totally unnecessary

Compared to the over-officiating in the NBA, NCAA, NFHS (US High School), I think the way in which FIBA applies its rules is much less punitive and a lot more straightforward. Having refereed using both NFHS and FIBA, I prefer using FIBA.