The amount of neg-g that we pull in AH is enough to permanently damage vision. Your point about GLOC from positive g's and the lack of something similar from negative g's is correct, but not quite on target to the point I was making.
Aircraft (both real and cartoon) are capable of pretty high negative G but physical damage is unlikely unless you're bouncing off the interior of the cockpit like a pinball. Typical aerobatic pilots (with good restraint systems) regularly do -3g with spikes over -5g during outside loops and we don't see eyeballs falling from the sky during airshows. It sure as hell isn't comfortable but you get used to G's (both positive and negative).
Actually, let me add to that last sentence. Negative G is never comfortable but once you're used to positive G it can be exilerating and it feels great. Like another physical activity I can think of, you get sweaty, exhausted, and sassified.