The problem is, the oil tankers are single wall, shoddily maintained, probably contaminated with all sorts of nasty stuff and with barely any history records. These things are effectively floating time bombs - assuming you can find a buyer for the seized cargo given the lack of paperwork in the first place, you need to sell the ship for scrap because it's nowhere near seaworthy (remember: single wall, no Western insurer will handle that), and that costs a loooot of money if you are a Western country and can't just haul it off to Alang [1] or whatever place and let others deal with the fallout.

[1] https://www.bbc.com/news/uk-56196069