It may be their primary residence, but apparently it is a violation of the lease agreement for it to be a primary residence.
But they are retired, right?
They are primarily on vacation :)
No one owns their land - only the structures.
...and will be billed accordingly
(wonder how much they pay in property taxes)
That’s irrelevant, unless the Feds got an order of eviction from the courts on that basis. It sounds like they did not, which makes this an illegal eviction. The residents can easily get a court order to deliver possession and are pretty much guaranteed to collect some damages as well.