It's only weird because you are so used to dice based combat. In video games, for instance, aiming where you think the enemy is going to be is the default way of doing things. Which is why things like stealth powers are common, but still foiled by higher level play in shooters.
In video games, you can see the terrain that you are firing at. In that case, I think that normal disadvantage works fine.
Try doing that in a video game when the screen is totally pitch black. We are talking the case where the PC cannot see at all.
Have you ever fired a weapon with a blindfold on? There is a major difference between firing at a spot next to a tree 100 feet away that you can see and firing at a spot next to a tree 100 feet away when you cannot see anything at all.
But, prove it for yourself. Go take a dartboard outside and put it in an old chair. While standing at the chair, put on a blindfold, walk 8 feet away, turn 180 degrees and knowing the elevation of the dartboard, throw a dozen darts at the inner circle of the board. See how many hit the inner circle. See how many miss the dartboard completely.
Now do the same thing, but cover the dartboard with a large sheet of paper and do not put on the blindfold. How hard is it to hit the inner circle when you can see the edges of the dartboard, but you cannot actually see the inner circle?
When one is estimating where an opponent is and firing at the estimation point that the PC (or NPC, the rules should work the same for both) can see, the disadvantage rule works good. When a PC is totally blind, the disadvantage rule works good if the foe is in melee, or if the foe is making enough noise that the PC can generally pinpoint his approximate location.
But if a PC is blind and he cannot really hear a given foe at range over the noise of combat, the normal disadvantage rule gives him way too much of a chance to hit his foe. So yeah, I'm totally ok with my DM putting in a house rule for this scenario.