Even when ask to join network is off, as long as wifi is on, it still scans for "known" networks which impacts the battery life negatively
As 2 other people have stated in this thread, when wifi is on and ask to join is off, the phone disconnects from wifi networks after 60 seconds of being locked and only scans for a split second for wifi once its unlocked. This is noted by Apple also. It's been that way forever now. If your device is draining battery, then you should do a full restore with no backup. If the problem persists, take it to Apple.