The legal grounds: The oil was shipped by a US company in violation of US law. American companies can’t do business with an organisation that the US government has designated as a terrorist organisation. Thus American authorities siezed the ship and its cargo.
This is like the people who repackage and rebrand LibreOffice and then resell it for $10 on the Windows Store to gullible users.
And the worst part about that is that it doesn’t even break the law.
You’re technically supposed to use a human cashier lane if you have a lot of groceries. At least in the USA, it’s pretty common for self-checkout lanes to have “15 items or fewer” signs.
That’s because even a grey market Windows key costs US$20 nowadays and that’s over ₹1,600. For comparison purposes, the largest Indian banknote is ₹500.