...Except Bajorans enlisted in the forces of an enemy
of the occupying force. In analogy, Cardassians should be enlisting in the Dominion military, rather than the UFP or KE or RSE ones.
Did the Dominion leave the Alpha Quadrant? Now that they had Odo and the cure, they would have no real reason to withdraw. And as far as we know, they never promised to do so anyway - the treaty witnessed at the end of "What You Leave Behind" was only about stopping the fighting. For all we know, there now exists a little corner of Dominion in the Alpha Quadrant, perhaps a star system or three, and angered Cardassians are free to join the mighty Dominion war machine there if they consider this will one day help liberate Cardassia from the occupation.
...It's not as if the Bajorans who joined Starfleet would have thought highly of that organization in absolute terms, either. (Sure, it hadn't exactly bombarded Bajor to tiny pieces, but whether that's a distinction worth noting is debatable.)