So go ahead and suggest an alternative. That is, one where it is neither day nor primetime in America, Europe or Asia but still is 8 consecutive hours in length. Good luck.
Could vary the maintenance windows. So they are set at local time in the early hours of the morning for each of the different regions.
Now for a time I thought that all the servers were in the same place and so it made most sence to take them all down at once. But then Ping would be the same in each place but this isn't the case, I suspect the European Servers are in Ireland along with the CS centre. While the American are in Austin. We know individual servers can be brought down when they have a problem and while it may take more work it would be possible to bring the different server banks down at one time.
Though whether BW does this I suspect depends on how much they value their Non American Customers. The other option would be to have a 3 week rotating restart so it was in different people peek playing time each week. Once again that depends how much you deem the downtime in someones prime time to be undesirable. If its an inconvenience then there is no reason that you can't inconvenience different groups of players each week. If it would be deemed unacceptable to have down time in NA prime time then perhaps its also unacceptable to do it in others prime time.