Patching is normally considered part of scheduled or emergency maintenance and therefore doesn't count against the SLA for uptime. This is fairly standard in the hosting/ISP world.

So much of this can be automated now that it is not a problem. As a provider myself, I allow customers to pick their patch day/time. They can even manually push patches themselves and be present to test when the service comes back up. Proactive maintenance(datacenter, networking, hardware, OS, and appptack/utils) should be considered a way of life these days if you're a provider. If customers don't understand or agree with that, then there are plenty of providers who don't keep up-to-date offerings that they can migrate to.