GAE has actually had 100% uptime for the last year. http://googleappengine.blogspot.com/2012/01/happy-birthday-h...
Blanket SLAs are not normally worth the paper they're written on. Look up the breach-compensation for your favorite TOS-SLA sometime.
Publishing the uptime history is a much more honest approach. Kudos to stripe for doing just that.
I'm questioning the comparison with other unrelated systems that are much bigger than them, especially when they don't publish a SLA themselves.