1: https://twitter.com/levelsio/status/1600232199243984897
2: https://twitter.com/levelsio/status/1600246753348882432
3: https://twitter.com/dannypostmaa/status/1600372062958538752
So if someone from US actually wants to go through the trouble to save $3, well, at least they're unlikely to be a bot.
It worked largely pretty well to keep out the trolls; as it turns out, a very low amount of people bother trying to troll others when it means that they get hit with an account ban and signing up again means paying the entry fee again.
You could probably also see it as the reason that while SA culture is uh... pretty toxic, it still largely managed to be fairly consistent and polite to each other (towards other communities... less so). Take away the 10$ signup fee and what you get instead is 4chan (whose original culture was a wholesale copy of SA at the time, since it was made for SA users after moot got banned from SA).
You could to some extent make an argument that gatekeeping poorer economies is one way to prevent those bots from signing up. It's not one I necessarily agree with, but it is one way to mitigate the spam.
My solution would probably be to permit users from poorer countries to request a signup from someone else at a discount appropriate to their economy using an invite chain. That way you can still offer a fair way for users from poor economies to engage, whilst allowing for easy banning of spambots simply by treebanning the original inviter if you get the spam issue.
The penalty scales with the number of bot accounts, but even Bill Gates can only drive one automobile at a time.
The USPS has a fleet of many (hundreds of thousands?) vehicles, so their capacity to ruin it for everyone is much larger - but their potential liability from fines is too. So they treat it very seriously.
The poster is saying that spam is more like the USPS situation, where single entities control thousands of potential infractions, not the rich individual.