"Where it gets interesting – and pricey for Amazon – is the "Portfolio Package" tier, which will give startups vetted by accelerators and funds between $1,000 and $15,000 in AWS Promotional Credits"
I'm not sure if "pricey" is the word. If they already have you on the hook, then I'm not sure if they are loosing anything at all, they're probably just making less of a profit. Remember, this was a company that for years was willing to sacrifice profits to just kill off competitors.
"Second-tier cloud provider Rackspace has its own giveaway program, but the $50 a month credit doesn't amount to much."
Maybe it doesn't amount to much, but choosing them and helping the competition stick around should amount to more than $50 at end game. No, I don't work for Rackspace, I just don't like Amazon.