The ruling was 6-3 with Clarence Thomas, Samuel Alito and Brett Kavanaugh dissenting.

Kavanaugh's dissent is particularly peculiar as he wrote 'refunding tariffs already collected could be a “mess” with “significant consequences for the U.S. Treasury.”'

So, the justification is that undoing an illegal act is going to be unwieldy for the govt, so presumably, as a corollary, the govt must be allowed to continue doing illegal acts. This honestly reads as a blanket support for Trump personally, than any reasoned legal argument.

I think it was more that they felt that the judgement should include instructions to dismiss any remedial action, not that the actions should continue. Without reading the dissent(s), I can't really say...

In the end, the people who bought products that paid more won't get it back... and who will receive the difference is the middle-men who will just pocket the difference profiting from both ends.

I think this is normal for the supreme court, I've heard that they largely upheld abortion in the 1992 case because they thought it would be a mess to undo, even though they thought the original ruling was unconstitutional.

That's Kavanaugh for you.