That's right John, and thanks for pointing it out.
We figure that if we've hired someone to work as a PM, then they'll have to know the rates of anyone (contractors or otherwise) they're managing on their project.
So we're forced to limit our concern to restricting contractors from seeing rates of other contractors on projects other than the ones they manage. (We also have to ensure that contractors cannot see the task cost info for projects in which they are Members or Guests, for exactly the reason you mentioned.)
Cheers,
David