IT MAKES ME SOUND LIKE I'M SHOUTING ALL THE TIME
The voice in my head gets louder when I read something in all caps. Its annoying, although I can quiet it down to regular volume if I try.
But, when I read something without caps, the voice in my head doesn't get quieter, so it doesn't bother me a bit.
Maybe the people who don't use caps are annoyed by having to reach all-the-way over to the shift key. It just happens automatically for me so its no problem, but if they have to think about pressing shift everytime, I could see how that would be annoying.