The idea behind restoring base mana is that it won't massively outscale spellcosts, it's easier to keep base mana balanced, than maximum mana.(Because base mana doesn't scale with gear, talents etc)
The point behind LoH is to restore health anyway, the mana is just a bonus. Also, it'll usually be used on targets without mana, or for whom mana is trivial. So you might as well remove the mana aspect of LoH, and nobody but whiny PvP'ers will care.