The $5B isn't a gift. Amazon is buying shares for $5B, and they're getting a spending commitment. I don't have any insight into the agreement, but on a ten year $100B spending commitment, I would expect $5B to be spent in no more than 3 years, and likely sooner.

In my reading, Amazon is giving $5B of usage credits in exchange for shares. If Anthropic works out, it's a good deal for Amazon. If it doesn't, they lose on their invesment sheet, but they got ~ $5B in revenue, so it looks good on their operating sheet. And it helped justify a build out that they can sell to others.

For Anthropic, this lets them operate for more time without having to make numbers work. If Anthropic works out, they'll figure out the $100B commitment later. If it doesn't work out, it's not their problem.

It's probably faster to build up amazon's capacity with amazon's money than to build owned capacity with someone else's money at the scale they're looking to build out.