Because its a DSLR body size camera after all, causing many to refuse them. You get rejected with those in most venues in Europe.
Ahhhh, Europe. Here in the US, I just show them the camera and inform them it'd non-professional...the lens doesn't come off...and they say OK. They have to draw the line somewhere, and they draw it at removable lenses.
I've seen security refuse people with cameras like this though ... because of what they perceive it to be. And unfortunately, to the untrained eye, a camera with an extended full-sized lens looks like a professional DSLR camera. And no amount of explaining that the lens doesn't come off is going to change their minds. So now you're stuck having to lug the camera back to your car or checking it in at security. I haven't been to a show in the States in a couple of years though, so maybe things have changed.
In all of my experience, and I bring the FZ70 to every show I go to, in the U.S., they established the guidelines as follows: Non-professional cameras only, as to be defined as a camera with removable/interchangeable lenses. When I'm going through security, I always offer to show them the camera and as I'm revealing it, point out that that it's not a professional camera, while pointing out that the lens is permanent. They haven't denied me entry with it yet. Good to know though,, if I find myself in Europe for a show.