I suspect that's so you don't type it wrong, copy it, then lock yourself out. I have typoed passwords before, very annoying when you know the password and just need to figure out which permutation it is.

I loathe sites that disable pasting passwords (I'm looking at you, Apple!) as it makes entering that random 24 char string generated by Keepass an exercise in frustration.

Worth mentioning is that battle.net passwords are not case sensitive.