7 casting dice vs. the average 3 body or willpower is still quite dangerous. Especially from a public viewpoint.
A gun shoots you,, and kill or knocks you out. Mages can torture you, interrogate you, fry you, electrocute you, turn your skin to goo, rape you, make you rape someone else, force you to commit suicide, put thoughts in your head, read your thoughts, look like you, look like your significant other, etc.
It's the vast amount of things the mages can do that makes them "special case" dangerous to the public. Which is scarier, getting shot and killed, or being forced to take a backride seat as you murder and rape your wife and kids then take a swan dive out the fifth floor?
In basic Shadowrun, it doesn't make sense for mages to agree to contracts that requires them to be frontline fighters, unless, of course, the corp is paying them substantially more.
Depends on the mage doesn't it? They're people too, and they have to take the jobs available. Sure, Mr. Superace that's excelled as his job for the past 5-10 won't be taking a job in the danger zone, but what about Mr. Crappypants that's already botched two high paying jobs and is having trouble finding work that will hire him.
Just because the Mages are rare and special does not mean they have a magical free pass. They're normal people with an abnormal skill, they've got outside lives, work habbits, work ethics, and pasts. Any number of things can be a reason they're working as Joe Mage Security at the Break In Gallery.
Sure, the Mage probably isn't going to be walking patrols with the other guy. It makes more sense to put him somewhere high with a good view of the grounds, or preferably in a security room that has a fiber optic web to get views of the whole compound/grounds so he can cast from the safety of a locked sealed room.