And when they charge USD they should learn to correctly format USD-denominated amounts...

You have a point but is there any currency notation in which 4,20 can be equal to 420 - thus omitting the "," ? When for example I see 4.20, as a European I assume "." is the decimal separator and not a mistake thus the number 420

If I have never seen a European decimal comma I would certainly be very confused -- most people aren't familiar with foreign currency notations and a hundred separator assuming that's a thing (most people aren't equipped to answer the question of whether it's a thing, me included) would be as foreign as a decimal comma if not less when you're familiar with comma as a thousand separator.

(I was confused for a second anyway but tipped off by the outrageousness of $420.)

Edit: Since this is Hacker News, let's talk about programming languages. 4_20 is the same as 420 in Python (3.7+) but not 4.20; 4'20 is the same as 420 in C++ (14+) but not 4.20.

If you haven't encountered the other system, you might expect it to be consistent everywhere. Or otherwise you might expect most people to understand either version. I don't think it's a big deal.