That's my guess--either they viewed it a personal attack, or else they were concerned that IdiotBoy (the original letter writer) would do so and they'd wind up with a flamewar. (The latter possibility, in my view, would be cause to edit the NEXT letter in the chain rather than this one, but....)

Still, even stipulating that the above is valid, ISTM that a bit of judicious editing with a rather lighter hand could've taken care of that and still left the meat of your reply intact, rather than the wholesale gutting they did. :-/