Once again, these mythical Starfleet Marines are Starfleet Security. Just like we have someone wearing a blue sciences uniform specialize in biology, I think we can have gold security specialize in ground combat.
But they are not called Starfleet Marines. They are a part of the already existing Starfleet Security division. Starfleet is the Federations defense agency, and if Starfleet personnel are defending a colony against a Klingon attack or holding a communications array from the Dominion, then they are a part of Starfleets combat division, known as Starfleet Security (providing Security, one dead Jem'Hadar at a time).