My take on this is, there is always a certain level of censorship in every country, under every government and platform. Either you truly don't censor anything, e.g. allow ISIS propaganda on your media as well, or you acknowledge that somewhere a line has to be drawn, and extremism preached by the western right or left is still extremism, just as what ISIS preaches is extremism.
Otherwise it'd just be hypocrisy and double standards.
In the mid-20th century, the US equivalent of "ISIS propaganda" was "Communist propaganda", and Communist literature was in fact legal in the US and widely available in libraries, although not generally broadcast on TV. This was, in fact, held to be the major distinction between the US and the USSR: the USSR "acknowledged that somewhere a line has to be drawn", and the US did not.
Otherwise it'd just be hypocrisy and double standards.