The historical precedent is very old.

Whatever happens, we have got the Maxim gun, and they have not" is Hilaire Belloc, 1898.

More recently in the 1980s. ITAR, the international traffic in arms laws, restricted European digital telephone switch sales to the USSR and allied nations. The US used this to secure contracts for Rolm with special permits. I worked briefly with a company in the UK which was prosecuted for selling Vax 11/780 parts behind the iron curtain, in this time.

And of course the entire history of Netscape and encryption leading to Mozilla/Cryptzilla in the early 2000s, export restrictions on Kerberos from MIT, the magic vlsi design tools, you name it. The BBN butterfly router, sent to UCL in London in the mid 1980s had export restrictions on who could operate and see inside it.

China is building its own euv capable vlsi, precisely because of the export restrictions. AMSL isn't allowed to sell them the fab machines they sell to TSMC.

What justifies this is force majeure: we have the Maxim gun, and they don't.

Because they can. The nations with bigger economies and tech lead will always be able to impose their will on the others. All states are not created equal.

I don’t understand why people use international law as their reasoning for why something is wrong or right.

Law and morality are only indirectly connected - something can be entirely lawful yet completely immoral, or illegal yet moral.

And international law isn’t like national law because there’s no world police - break international law and you get some angry open letters from legal experts, countries that don’t like what you did complain (which they would do regardless of legality), countries that do like it don’t complain. If you’re a world power there’s nothing they can do about it.