I agree that offering this makes sense for AWS. What I'm not so sure about is whether offering this makes sense for YC.
(BTW, Tarsnap isn't in any incubators/accelerators, but if someone wants to throw a bunch of AWS credits at me I'd be happy to have them!)
You could probably apply for the Amazon accelerate program and get $15K from them pretty easily.
The $15k of credits are only for "startups in select accelerators, incubators, co-working spaces, and other startup organizations".
Hosting companies give away free credits like candy. Don't let the lack of incubator stop you from asking. I've gotten up to $10,000 in hosting credits just by emailing someone and asking for it. No contract or anything, they just added credit to my account.
Unfortunately I cannot disclose the numbers and the companies, but they are way larger than $5k
$5k in credit for hosting probably has a marginal cost of $0.
On the other end, if you take AWS offer and start using their services, you'll probably won't leave them anytime soon, particularly if you rely on provider-specific systems (SNS for instance).
Startups get a nice discount, hosting providers get an opportunity to supply a future successful startup and very quickly win back the $5k...