That's just stupid.
No, it's freedom of speech. Defamation is saying untrue things about a defined organization or a person, which will result in some financial or severe reputational harm (unless it is done for parody). We tolerate that minor restriction on free speech for the specific. However, for statements made against some sort of birth-characteristic group or an entire religion in general, we allow people to have their freedom of speech... mostly because it is very difficult to measure the harm in that case, but also because we want to curtail the restrictions on speech. We will allow remedy for the lies about the specific, but not those about the general. Freedom of speech is the most important right we have. Any restrictions on it should be of the most narrow.
Otherwise, saying something like Muslims in general don't care when terrorists blow up Israelis (with the implication, of course, that they implicitly support it) could be considered 'defamation'. I'm sure you wouldn't like that.
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