One would think that they would at least have to inform you at the time of purchase that you aren't allowed to bring in an SLR camera. As for actually confiscating your own property, I highly doubt that's allowable; saying you can't use it, sure, but not taking possession of it themselves. Only the police have that ability, and only when there's sufficient reason to do so (e.g. arrest). I know, for an absolute fact, that no one has the right to confiscate your property (regardless that you are on their property or not) here in Canada. (What they can do is refuse you entry, but if they didn't tell you that you wouldn't be allowed to bring an SLR at the time of purchase, it's essentially an a posteriori change to their agreement to provide access to the concert for a certain fee, in which case you have every right to demand a full refund.)