I believe the "keep alive when screen is off" setting is specifically for the tilt function.
One possible workaround, depending on how you use your tablet, is to have your tablet set to turn off wifi when it goes to sleep (using Tasker or one of the free apps that do this, or your OS settings themselves depending on which OS.)
Of course, if you want to keep getting other kinds of notifications that arrive via wifi when the device is asleep, this won't work.
Most folks seem to want the mic to stay on with their screens off when using "always on" mode.