People generally won't argue when good ideas are cloned between competing products, and praise the competition.
In the same way it's ok to write a theatre play in which the love between two people is contrasted by their families, but putting "wherefore art thou Romeo?" in the dialogs would seem bad.
Now we can answer why it is that way. Why do these comedians shun peers who steal jokes? Well the first reason is that it leads to crap. So, this sometimes happens with movies that are intended as comedies: ten different creative voices all try to push the jokes in a certain direction, and what you get is not extremely funny to 10% of the population -- which would be success -- but is only a little funny to 100% of the population -- which is a failure. So by taking someone else's joke and mixing it into a hodgepodge of other material, it stops being so funny as when it appears in the context that it's supposed to be in.
But I think a second reason is that we nurture ideas. It takes effort to turn an absurd concept into a good joke, and it takes effort to take an abstract idea like "coding with other people" and turn it into GitHub. When a comedian tells someone else's joke, there is a false pretense that they have spent all of this work to develop that joke for themselves -- and they haven't done any of it. When Eddie Izzard says, "This is my friend's Christopher Walken impersonation, and I'm nicking it, and he knows that," he's disavowing this aspect of the joke, and therefore it's more acceptable and can be used in his routine without too many eyebrows raised. The frustration is with people who steal almost every joke and achieve success with that -- not with people who steal one or another joke when they think it deserves to live on.