The quote in the title isn't in the essay I'm about to link to, but we've all heard "he's just doing his job" a million times as an excuse for immoral behavior. I couldn't care less about the manufactured Don Imus fiasco, but Constance L. Rice trots out the "he's just doing his job" argument in an otherwise excellent reframing of that situation.
But there's also no basis for firing him or ending his show. Firing Imus for racist riffs would be like firing Liberace for flamboyance. It's what he does.
Setting aside the specifics of the Imus circus, I find it curious that the question of whether or not a person makes a living off some type of behavior is a legitimate justification for actions that would otherwise be unacceptable. I've heard drug dealers excused in this way, along with all sorts of other ne'er-do-wells. The sequence in Aladdin in which we're introduced to the title character features him stealing food, evading the authorities, and singing "One Jump Ahead":
[Aladdin:] Gotta keep
One jump ahead of the breadline
One swing ahead of the sword
I steal only what I can't afford
( That's Everything! )
One jump ahead of the lawmen
That's all, and that's no joke
These guys don't appreciate I'm broke
[Aladdin:] Gotta eat to live, gotta steal to eat
Otherwise we'd get along
Judging from the accompanying escape sequence Aladdin was an able-bodied fellow, so it's not at all clear why he couldn't work for food like the crowd who despised him and the guards who chased him rather than "working" as a thief.
Imus, Howard Stern, and their ilk make money from paying customers, but their customers are essentially paying them to be rude, arrogant, useless jerks. Most such people are rightly ostracized by mainstream society, but apparently the profitable ones can be forgiven.