Assuming you have been driving the vehicle it's unlikely the surface
that is used for braking is rusted.  Any rust gets rubbed off when you
brake.  My guess is they are trying to screw you.  Brake jobs are
often an easy source of revenue because they can sell you on FEAR of
bad brakes if you don't let them replace/repair every imaginary flaw.

If they are trying to remove the rotors to resurface them, it is
entirely possible.  A proper brake job includes either resurfacing the
rotors or replacing them with new ones.  It is completely common for
these things to be difficult to remove due to environmental rusting.

If you want a proper brake job either find a mechanic that has an
on-the-vehicle resurfacing machine or purchase new rotors.  Personally I
would rather spend my money on a new set of rotors...

Good luck,

Steven