"Except that these companies have policies which the employees have presumably read and agreed to."
Private contract does not trump the Law of the Land(*).
The previous poster said, 'In the UK' that would probably be illegal.
I can add to that that in France it IS illegal, and from article I read about Sweden, Norway, Germany, to name a few, I believe that it would be illegal there too.
In the US, The legality of each would hinge - I think - on who is the owner of the device is.
It is not that obvious that a remote wipe on the personal device, especially of personal data, would be uphold as legal by the court.