Sorry but what a lot of tosh...
BASSA is part of a trade union and comes under trade union law. If it is breaking that law and is discovered, the defence of "oh, but we were rumbled by a non-member" simply isn't going to cut it. The law is the law and if you can't run a trade union legally then you face the consequences.
If you came across someone you believed to be burgling a house, but it wasn't yours, do you somehow magically lose the right to call the police?