To fix constant criticals, Valve released a new random crit system which requires players to "fill a bucket" by shooting before they can receive random crits.
It's possible to "store" random crits by filling up the "crit bucket", which can then be used at will by a cheater. Melee weapons do not follow this crit bucket system, which allows them to crit 100% of the time with cheats.
This cheat does not work on servers with tf_weapon_criticals and tf_weapon_criticals_melee set to 1.