I feel the thing we all need to realize here is that it's not just the material in question, it's the length of exposure to it.
Many people may be allergic to things, just we pick them up, move them to another location and put them down.
Someone mentioned coins, well, generally you pick up a coin, and give it to someone else. the contact time is just seconds.
The BIG difference here is that, unlike many many other things, an iPad, or perhaps an iPhone are things that could easily be in contact with your skin for perhaps hours at a time.
If you think about it, other than cloths, many things we hold onto for long periods of time are plastics. A Game Controller being an excellent example of something you may well have in your hands for many hours each day, esp children.
A metal game controller may set off an allergy with someone. Or course for this type of use, grip and feel Plastics are better.
We unfortunately still suffer for the totally irrational concept that's been sold to us by some that Plastic is cheap and nasty.
Just because you CAN select a grade of plastic that is cheap and suitable for say green toy soldiers in a poor quality mold. Does not mean you cannot also select a very very high grade of plastic with many excellent quality's that is far better for mobile devices when it comes to strength and impact resistance.
Aluminium is cheap, common, and pretty unsuitable for many uses, we need to get over this silly idea that Aluminium is the best product always.