Great question — answering as the UphillTech team:
I'm not sure you looked very hard :-) From the offical FIBA rules , Rule 17.3.1: The player taking the throw-in shall not: [...] Cause the ball to enter the basket directly In the NBA rules , this is covered by Rule 10, Section III: A thrower-in shall not [...] throw the ball so that it enters the basket before touching anyone on the court
Full answer by Philip Kendall: https://sports.stackexchange.com/a/12420 (CC BY-SA 3.0).
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