Most big venues will check for contraband like cameras on entry to the building. If you're carrying a camera they will offer to keep it for you until the end of the performance in their "confiscation room". If you don't like it, you don't get in. O2 springs to mind.
Smaller venues won't have the resources for that. Leaving it up to an usher to confront a paying customer is risky from all sort of points of view. Its bound to cause more disturbance, and any action taken without the consent of the offender leaves the venue open to legal action (assault, criminal damage, as above), never mind the T&Cs.
(Breach of contract versus criminal assault? No contest! Wasn't there once a Thin Lizzy gig where someone was hassled (strangled by his camera strap!) by security for taking pictures so Phil Lynott stopped the concert, reprimanded the bouncer and told the lad with the camera to take as many pics as he liked.)
As far as the possible copyright infringement goes, the only recourse the copyright holder has is to pursue you through the courts for damages. The copyright holder is unlikely to be the venue. The reason why the venue gets excited is because the contract it has with the performer and/or the copyright holder may require the venue to prohibit photography. People subsequently taking pics means hassle for the venue from the artists' legal team so in that case the venue will do all it can to prevent photography from happening. Most of the time its only indirectly a copyright issue. (Its like sportive organisers and helmets.....!)
This is complicated enough, but now add in a children's performance and irational fears of paedos using cameras, and other people thinking they have personal rights not to be photographed, and now try explaining all that in words of one syllable! No wonder people get confused about it all.
None of this justifies anti-social behaviour but just stealing a few snaps needn't be anti-social.