my (loaded) question

what constitutes screener/tso discretion regarding the 3.4 ounces of liquids restriction and why would it be allowed at all with the exception of medical and infant needs? we've all read, heard and perhaps even experienced situations where the infamous 3.4 ounce liquid restriction has been (for lack of a better term) "waived" but it still boggles my mind that with all the brow beating we get that "only liquids 3.4 ounces or less" will be permitted into the secure area.....