Each dealer can choose to service whoever they want. You can complain directly to Ford MoCo, although I don't think that would help too much as they aren't refusing service they have to perform (ie warranty work).
You could try and find out more about why they won't work on it, see if it's a real policy (ie printed) and review it if you choose.
Personally I've never run across this, and it's definitely not a Ford MoCo policy.
Sounds to me like they only want to work on 'current' vehicles, which would make me question their mechanics abilities.
Thinking about it, I have actually known a dealership (Vauxhall back in UK years ago) who would only do warranty work. That was because they were primarily a seller, and not equipped for extra workloads. They turned me away one day, explaining this (more or less), but did recommend many places to go.
If in doubt though, ask. It's the only way you'll get to the bottom of this.
