Interesting question. I assume piracy and smuggling and various other law breaking but I'm not certain. AFAIK the only requirement is a legitimate registration. Again AFAIK the vessels that were directly interfered with (ie by force) all had either falsified registrations or were flagged under countries that aren't currently in any state to actually manage registrations.
Then there's also their participation in what's been termed a shadow fleet, the associated falsification of origin of sanctioned oil, the accessing of ports where they otherwise wouldn't be permitted berth, the lack of insurance in case of environmental damage, etc. As I said previously, much more complex than the mass media portrayal.
As you say, the shadow fleet exists because of sanctions. In other words, because the biggest bully on the block is committing de facto piracy with their navy. Pretty much the definition of blocking freedom of navigation. Their insurance paperwork not being in order justifies their seizure?
That's a half truth at best. The sanctions in question are hardly unilateral, particularly in the case of Russia. The shadow fleet exists due to a combination of factors; dodging the sanctions is only one of them.
As I understand it (but I'm certainly no expert) the insurance paperwork isn't in order and the fleet not properly registered as a result of the general state of the vessels involved. The US is hardly alone in this - the UK has also recently taken to seizing such vessels that pass too close to them. But generally yes, if due to the risks no country wants to officially register a vessel and no insurance provider want to cover it then it seems entirely justified to seize it in order to protect the commons. These aren't pleasure boats we're talking about here, they're ridiculously large merchant vessels. There's approximately zero legitimate excuses for them to be flying a fraudulent flag.
Ponder for a moment why it might be that the countries involved don't want these vessels flying their own flags and don't want to extend them insurance policies themselves.
Freedom of navigation is a right of countries. Spanish ships have the right to freely traverse the seas next to Greece, etc. So ships have to identify themselves as belonging to a country so that they can benefit from it.
