Great question — answering as the UphillTech team:
The only rule I'm aware of that modifies where the defender can stand on a throw-in is 7-6.8.e e. In arenas or gyms where there is insufficient space along the out-of-bounds line for a throw-in, an official shall require the defender guarding the thrower-in to move back a reasonable distance to give the thrower-in an opportunity to make a throw-in. @bradimus mentioned in a comment that this particular game had a raised court that would have made it more hazardous for the thrower-in to back up compared to other games that appear to have similar amounts of space.
Full answer by BowlOfRed: https://sports.stackexchange.com/a/30262 (CC BY-SA 4.0).
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