Like Ged said ..... could be a number of different reasons why .... and i could certainly see venue owners using the "been sent an anonymous letter" as an easy way out.
But if it had been me that had been told that someone had wrote a letter saying things about my club and as a result was being stopped from holding anymore nights at that venue........ i would demand to see the letter!
Surely they would have to show it to me, wouldn't they?