Taking credit has nothing to do with it (except Frankie Boyle - that guy WILL say anything).It must be coz I'm old or something but for the life of me, I cannot see how anybody would wish to take credit over something 'funny' said about Madeline Mcann or anything of that ilk .... I find it hugely offensive on oh soo many levels and completely without any form of justification ... maybe I'm out of step with modern thinking but in this case, I'm glad I am because if you can find humour in a case like that then nothing is out of bounds and once you break down all barriers as far as humour is concerned, social amorality is close behind.
I know someone studying Journalism at University, she says that whatever content that is published or broadcasted or communicated to an audience in anyway is subject to laws. For example Jeremy Clarkson (Top gear) is the most sued celebrity because for everytime he mocks a celebrity he can be sued for deffemation (Dunno how to spell) but it means your attempting to damage that public figures reputation. So comedians CAN be sued for their jokes, but in the celebrity world it is frowned upon as some comedians persona is very blunt and they often relate their content to the latest news.
So celebrities are highlited as attempting to destroy a comedians careers, therefore they often do not press charges.
There are more laws depending on certain criterias, but in a brief sense, yes there is boundaries in a sense and they do face real life consequences sometimes.