[laacz]

I don't think that's the reason. 99.9% uptime is plenty. Uptime in SLA is defined as "Monthly Uptime Percentage", which is explained as follows (http://www.google.com/apps/intl/en/terms/sla.html):

"Monthly Uptime Percentage" means total number of minutes in a calendar month minus the number of minutes of Downtime suffered in a calendar month, divided by the total number of minutes in a calendar month.

For average month of length 30.4 days (~43776 minutes in total) that means acceptable downtime of 437.something minutes or 7 hours of combined unavailability.

Minding all of that, there is also a term "Google Apps Covered Services", which is explained as:

"Google Apps Covered Services" means the Gmail, Google Calendar, Google Talk, Google Docs, Google Groups and Google Sites components of the Service. This does not include the Gmail Labs functionality, Google Apps - Postini Services, Gmail Voice or Video Chat components of the Service.

So, legally speaking, there is no uptime guarantee for Google+.

[gojomo]

…that means acceptable downtime of 437.something minutes or 7 hours of combined unavailability…

You're off by one power-of-10: 43776*(100%-99.9%) = 43.8 minutes of acceptable downtime.