The traditional short-barreled "riot" shotgun always had a Cylinder Bore, or NO choke barrel.
Remington, and many other makers are now using the Improved Cylinder, (Slight choke) because it tends to produce tighter groups with slugs, and tighter patterns with buckshot.
Remington's Law Enforcement web site says the Improved Cylinder is used on the Police guns "To improve pellet accountability in Law Enforcement use".
In other words, to produce a tighter patterns to help eliminate stray pellets.
Remington makes the less expensive Express line of 870 shotguns with the Cylinder Bore as a cost savings, and the Marine Magnum is an Express-based gun.
Since the Marine Magnum was intended to be a home defense and boat gun, the open Cylinder choke may actually be a better choice, due to the shorter ranges it's likely to be used at.
At shorter ranges the slightly more open patterns will probably be an asset.