Sorry to sound so crass, but this is a native Warrior trait. You're saying it doesn't apply to a Great Axe? So wielding a staff + grip loses fencer? Makes no sense...
What Onorgul said. It was intended to give more versatility to warrior, to allow for the use of their one-handed weapon proficiencies, as well as to benefit some of the one-handers that sub warrior.
It has mixed results for the former, but it is certainly helpful for the latter.
EDIT: Of note, BST and BRD (funny, SE...) get a degree of Fencer as well. Also isn't available when dual wielding.