I don't think anyone is arguing about ideas in their purest forms. It is always against ideas manifested somehow, and ideas only manifest themselves (in regards to this topic) through people. Expression, speech, as in freedom of speech, is a manifestation of an idea, so it is not a pure harmless idea anymore. It is an action. It may derive consequences. One may argue that you should interject on the consequences, protecting the action of manifesting an idea, and that is fine. But I think it may happen in some cases that interjecting on the consequences is so much more inefficient, that some prefer to interject one step earlier and forbid an idea from being manifested at all.
It is like castrating an idea. It still exists, but doesn't multiply.