I was in the Halloween audience last year. The stage managers didn't
force the audience members to do standing ovations - we were simply told that they would appreciate it if we were to feel moved to give dances 'spontaneous' standing ovations.
To be fair to the BBC, these are free tickets and the audience is very much part of the show. As such, it kind of behoves them (us) to join in and do what is required to add to the atmosphere, even if that includes 'spontaneous' standing ovations. Ditto clapping along with the music over the opening credits, cheering every lift from the dancers accompanying the guest act (which I hated doing, since I think it's rude to interrupt the singer) and generally entering into the spirit of the show. We were reminded that the cameras might be on us at any point, so please to remember that and act accordingly.
It's a bit of a Faustian pact, but we felt lucky (and privileged) to be there, so were happy to play along with it all. It's an entertainment show, you know...
ETA: I've also been in the audience for Dancing On Ice, where we were asked much more forcefully to give standing ovations to EVERY act, and to boo Jason Gardner (not hard to follow that instruction). The BBC is a lot more subtle in its encouragement.