I mean - you look at the three of them, Gordon and Graham give constructive criticism EXCEPT when it's justified here (i.e. undercooked proteins, something that truly is inedible, cheating). They want the chefs to do better and succeed in their cooking, and not discourage them. When they're mean regarding undercooked foods, it's so the chefs know what can happen - and folks have corrected it quickly if they weren't eliminated (rare but has happened).
Joe is mean to be mean. I mean he has been critical of folks with handicaps too, and stereotypical of them (i.e. Christine last year - he was always questioning her). Sheesh, he can't even eat a dish without giving the stink eye. Even Simon Cowell wasn't that rude.
I wonder how many people have given up trying to be creative in their cooking thanks to his rather rude remarks.