When I get hand pain, it seems to be related to how much I have to tense up my grip on the sides of the mouse to hold on to it as I lift. I play with a pretty low sensitivity, so lifting my mouse in-game is important. When I try to do a full palm grip on a mouse, it's pretty comfortable for regular use but when I get in-game, I get hand pain pretty quickly because they tend to have a \ slope on the right side. That means I need to tense up quite a bit and "compress" my palm, bringing the left and right sides of my palm closer together, to get a decent grip on it, though that may just be because of how the mouse interacts with the shape of my palm in particular. Anyway, to alleviate that pain, I tend to use a more claw-type grip. I can keep my palm more open by flexing my thumb, ring, and pinky fingers towards the mouse, and that relieves the stress on my hand. Something like the Zowie EC series and Deathadder are nice for desktop use, but I get into game and I start to get pain on both sides of my palm, just beyond the wrist. When I use my KPM or g303, I go into a full claw/fingertip, and I can play for hours with absolutely no pain. Even though my index finger is flexed just like OP's on the Deathadder, I have never once, with any mouse, had pain anywhere in that finger. For whatever reason, using my fingers to grip, alleviating stress from my palm itself, is the most comfortable way for me to grip a mouse.
I want to say it has more to do with how we trained ourselves to use mice: some of you may have started with an IME or something similar, I started with a smaller Logitech mouse, and then another small-ish Logitech mx310. I've always had long fingers, so even though I was young, those mice were pretty small and I never really used a palm grip. At any rate, making categorical statements about how everyone should hold mice is a bit silly. Do what works for you, but don't assume it's best for everyone else too. The fact that you can't imagine it being comfortable doesn't mean it can't actually be comfortable.