Because 16 years old do not have a card with no spending limits, and with very low online spending limits. Most of those cards are even just for withdrawing

Spending limits don't particularly matter here.

AWS doesn't check if your credit card will be able to handle a $5k charge before letting you rack that up, and in fact AWS doesn't support setting any spending limit.

You just have to put in any valid credit card at all when you sign up, use AWS, and at the end of the month you'll have a bill. At no point does your credit card limit or a spending limit enter into things.

And again kids don't have credit cards

I got mine when I was 12, IIRC. Not a credit, of course, it was a debit card, but not all countries bother to differentiate between the two, it was just a “bank card”. And I believe it had a credit card BIN because all local banks did that to get more in processing fees.

AWS accepts debit cards.

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Nobody has a card without spending limits.