Sinewave, other than the call-center/lead-age explanation that has already been given, another possibility is that the network/advertiser wants to minimize their exposure to fraudulent affiliates. Often times, a fraudulent affiliate will join a new network and begin pushing an offer hard on a late Friday afternoon and through the weekend knowing that things aren't being watched all that closely.
Yeah, it sucks that the end merchant isn't watching 24/7, but in some cases, the end merchant is a smaller shop and doesn't have the manpower to be running compliance around the clock. So they take the safer route and say "no weekend traffic".