It is indeed extremely puzzling that even proud, millenia-old civilizations that just escaped the clutches of tyranny would ever willingly submit to such a diktat.
The Blood Oath being worthless in actuality is a given obviously, since no Horde focused story even happens without it being broken a half dozen times if you really squint closely at everyone's actions. In fiction, oaths of absolute loyalty exist only to be broken at the end of the day, unless you want a faction so uniform and copy-paste that it makes the current Alliance look dynamic.