I think the idea here is that if the payment for SLA breach is just "don't pay for the time we were down" or (as I've seen in other SLAs) "half off during the time we were down" that doesn't feel like much of an incentive on the service provider.

They have other incentives, obviously, like if everyone talks about how Google is down then that's bad for future business. But when thinking of SLAs I'm always surprised when they're not more drastic. Like "over 0.1% downtime: free service for a month".