You'll have to ask the M/T Skipper's master about why he was flying a false Guyanese flag. I understand that he has been detained so that may come out during the investigation. The vessel had never been registered in Venezuela so there would have been no reason to fly that flag.

Are we really going to obtusely pretend we can't think of a single reason on our own for them to have done that?

I can speculate as to motives for flying a false flag but so what? Any vessel flying a false flag is subject to seizure and isn't entitled to legal protection.

And what should the U.S. vessels imposing an illegal and unprovoked blockade be subject to?

If you believe in "might makes right" why not just be honest and come out and say that?