Courts deal with slippery-slope arguments all the time, and defeat them by establishing clear tests. There are numerous tests, for example, in common law regarding defamation: truth, object of defamation, malice, etc. Likewise there are tests that allow some exaggerations in commercial speech but not others.
The American tradition is that speech generally may be censored. For example, some books by D.H. Lawrence and Henry Miller were banned in the US as recently as in the 50’s.
No kidding. And the point is it’s slippery so you stay away from it.
When I think of these terrorist attacks, I don’t think of banning cartoons. I think about solving the problem, which is the behavior of terrorists.