There was no need for the Guards to even come out and question the photographer.
He was going about his lawful business of taking photographs, and required no permission.
If they thought he was doing something illegal on public property, they could have called the police, but they had no authority to do anything themselves.
If they have been sacked, they brought it upon themselves, however I feel that sacking was unnecessary, and some instruction was all that was required.
I think the problem is the nature of the opening gambit which was:
"You're not supposed to take photos....you're not allowed to mate."
I've been approached very rarely by security and when it has happened, their opening gambit approximates: "Who are you/what are you taking photos for?" And there really is no issue at all. If someone (incorrectly) stated that I wasn't able to take a photograph, I'd stand my ground too.