The cards aren't charged. You could sign up for several accounts with the same card, and I would imagine it's trivial to buy credit cards with $25 on them each or buy up old ones with less. These cards could also be stolen - one possible reason the information doesn't align in the article. Imagine using stolen cards for authenticating an account that doesn't accrue charges. It's genius.

This also illustrates the rules of efficiencies. If there exists a resource one makes available for a given purpose (signing up lots of developers for free accounts) then you can assume there exists others who will figure out how to get those resources and turn them into something else of value to them.

There's no such thing as a free lunch. There's also no such thing as limiting fraud. It will always be a choice.