The currency symbol's placement is a matter of convention, which varies from person to person, mostly influenced by what they're used to.
I'm attacking your pedantic attempt to reduce the matter to an inflexible rule.
Some English speaking countries prefix the currency symbol, so if let's say my audience is from Australia, and I want to make the gesture of adhering to their customs, I will prefix it. However, in other (most) scenarios, I'll write it the way I write it 99% of the time, in the postfix position. And that's ok. Everyone doesn't need to use the same customs, words, grammar, or even language. Regarding these things, the only thing we care about, at the end of the day, is being able to understand the other, and the other being able to understand you.
As you've noticed, I said _some_ English speaking countries. That's because there are a lot of people, who even though use a lot of English, only a fraction of that is with people from US, Canada, Australia or Great Britain, so the customs of English users from those countries become irrelevant.
There's customs and there's language. This is an obvious example where one does not have to entail the other.