This seems really dangerous -- it creates a huge incentive for companies to waste YC's time. There aren't many things early startup founders can do which are worth more than $15000/hour. Sure, you can probably filter out many AWS-credit-seekers via the application process, but that adds more work for the people who read through all the applications.
Have you considered either (a) refusing the credits, or (b) taking a small number and handing them out to the most "deserving" startups at the end of the day?