Here's a theory. If the site had an issue, they would be unable to do anything about it that day. Could be too much of a liability, so they shut it down.
I wonder how that would work, though. In general the accepted work week is Monday through Friday, so even if you did hire/pay the person on a weekly/yearly basis, you'd still have to specifically say to them that they're required to work on the weekend, and thus the Sabbath. I wonder if that's ok or not...
Or maybe I'm applying too much thought to it.