The USCG is detaining them onboard with threats and coercion of adverse legal action.
The government operates minimum security prisons and halfway houses where it is quite easy for detainees to escape. The detainees simply face a choice: stay or face adverse legal consequences. Not much different here.
The USCG has the authority and ability under Ports and Waterways Act to detain the vessels, secure them, and release the crew. The USCG is simply choosing to detain the crew and force them to continue working indefinitely without pay in involuntary servitude, instead of detaining the vessels.
Obviously, the crew cannot just abandon the vessels underway and let nature take its course, but neither can the government detain the crew onboard to operate the vessels without pay indefinitely.
Itās up to the individual judges in various jurisdictions where the vessels are located to determine where to draw the line. I think a judge might well order that the USCG must secure the vessels (or put new crew aboard) and release the crew within a certain time (and it would be a relatively short time, perhaps a day of two). I think itās also quite likely that different judges would be more, or less, sympathetic to the crew, and reach different decisions.
I think that having to publicly defend multiple writs of habeas corpus, and false arrest, section 1983 civil claims, and explain that to the public, press, and Congress, would put a lot of pressure on the USCG to bring these vessels to secure berths and release the crews.
Of course, the USCG could put their own personnel aboard to operate the Bouchard vessels, or hire a government contractor with commercial crewmen to do it. There seem to be enough marinerās looking for jobs, and government contractors, like Chouest, Crowley, Liberty, Seaward, etc., available