Actually, lots of people have nickel allergies, and do not find out unless they get cheap piercings, or wear cheap jewellery. I'd imagine it's either that the nickel isn't in the hand long enough to irritate it, except in the most extreme allergic cases. Or it could be that the skin on the hands is tougher and thicker than the skin on the side of your face, so it takes more to agitate it.

But that's just my non professional opinion.

Maybe these idiots should just stick to plastic phones; or if they must have a phone with nickel parts, put a skin over it. 

It seems pretty unlikely that anyone could be allergic to nickel and not know about it - the aptly named US 5-cent coin is 25% nickel, and surely anyone (within the US, at least) using a cellphone has handled at least a few of those....