Even during the war though Starfleet respected individual freedoms. Garak always had the ability to just walk away. If they attempted to coerce him he would have been savvy enough to avoid the coercion. Garak was only ever going to work for Starfleet on his own terms.
And besides, Cardassians were not enemies of the Federation at that point, and by the time they became enemies, Garak had proven that he was against the Dominion in all ways.