Mick, you are right, it is getting totally silly - that and the general attitude of security guards. The suggestion that they have the right to get you to delete the images off the memory card is just wrong. The police can't make you, so how can a security guard?
To be fair, I do think this is a fairly rare occurance - I'm never been stopped by anyone - and I take shots of my kids in the park. Though I am aware of the other kids around and avoid having them in the shot as well. Not that I legally have to - but I don't want pictures of kids I don't know anyway!