I blame whoever at Bethesda wrote it and also whoever thought that was an acceptable plot point/ending and didn't ask for changes (which I guess is Todd Howard but then again he wasn't the director for FO3). They are so bad at actual logic and RPG design.
Entirely fair. Fawkes is a generally great companion/tank/storage unit. The blame falls on the writers' lack of imagination (and appropriate dev time). Not to mention Todd "it just works" Howard.
edit: even if it wasn't under Todd's direction, I'm still generally annoyed with his shenanigans. More so since eventually, I will wind up playing the next TES and Fallout entries. Presumably after a year post release and on a reasonable sale.
Just a reminder, this is only implemented because of a game breaking bug that would occur when you tried to send him in to do the job. I remember because I encountered that bug; I bought the game disk day one and didn't update it until later. You literally had to pick a different option because if you had him do it, he'd be like "sure thing that's a great idea," and then he'd walk in there and flip the switch and then the game would hard crash to desktop.
yes. the super mutant is immune to radiation, but you can't send them in to do the job. apparently you originally could but it got patched out because of a game breaking bug.
which is weird; Bethesda games are known for their polish and general lack of bugs.