They minimally do simple server restarts to make sure no memory leaks are building up and overwhelming them. In other cases, they put in hotfixes and other maintenance actions. It's a standard practice. That's planned downtime. For the US, this is 4 AM PT and 7 AM ET. It's the least peak hours. Any earlier and that cuts into the Pacific Time late-nighters. Any later and that cuts into the Eastern Time early-birds. As you can see, it's about as ideal a time for standard daily maintenance as you could possibly pick.
The Daily Shutdown is completely understandable. This is one of the biggest online games I've ever seen and it has hundreds of users logging on every minute of the day. I personally think that's awesome so I totally understand why they need time to be able to upgrade, change, and fix problems we report, and none of us should be upset about that. We should just be happy that they were nice enough to make it free to us WITHOUT any ads. That's just my opinion though.
