The USCG has far reaching and 24/7 monitoring capability of all Marine channels, perhaps they are cognizant of this and want to avoid being overheard by the fish cops.
The Coast Guard doesn't monitor all the VHF Marine channels. They are use a handful of the them and with the radio traffic on Channel 16 during the boating season would be hard. Mostly channels 16, 22A and whatever working channel out of 21A, 23A and 83A. There cutters and small boats would have a better chance of being on other channels than the Sectors and Stations.