It's easy to get a Visa/Mastercard in the US. It gets a bit trickier in some EU countries. Then the further from the west you go, the more complicated it gets, all the way down to impossible if you live in a place that the US isn't on friendly terms with (like Iran or Russia).

If you auto-assume everyone can pay any amount online (even if it's a refundable $1 for verification purposes), you're gonna cut off access to a lot of people unintentionally, while only raising the bar a little bit for spammers.