I don't know what the policy on MMORPGs is, but in most internet business, uptime is, in fact guaranteed.
Our company buys bandwidth from an ISP, and depending on the SLA (Service level agreement), we were guaranteed 99.5 or 98% uptime. If the company failed to meet that (measured over the course of a year), we didn't pay.
Also, our company had to provide similar guarantees for the websites we produced. While contracts varied, it was standard to assure a client that his site would be up 95% of the time, or service/maintenance fees weren't paid.
Brodrick of Caitsith, you are totally right in saying that we've accepted the agreement, and there's not much legally we can do about it, but it's certainly not standard practice in the broader industry.