Should they avoid doing that because it’s working really well at putting their opponents in a bad spot, while costing them almost nothing?

Those ships are bearing goods from (or taking goods to) countries that are hosting US forces attacking them. They’re valid targets, and blockading their shipping… I mean, the US does that to countries that haven’t even helped attack us, seems insane to suggest it’s somehow a foul to do that to countries that are helping attack you.

Running a blockade is a risky proposition; it's not something that happens by accident.

A lot of these "neutral" countries either host US military bases, US companies, or are otherwise aligned generally with the US.