It's not a slippery slope argument.* It's not that we fail to comprehend the bias of the Daily Mail.
It's that, as reported, Rhalkos seemed clearly support what happened. As reported. Now, if you think, "Well, that's a preposterous story, and I seriously doubt it happened in anything like the way they say it did. But if I were to learn it really did happen, I'd be steaming mad and I'd demand that someone had better be held accountable," -- well, okay, then. Then we'd be on the same page.
But ... I just don't get that impression. The general impression I get from most Europeans I've met, including Brits, is that they simply don't value freedom of speech to nearly the extent that we do in America (on average, but overwhelmingly, regardless of political persuasion). Our reason for support isn't a slippery slope question (though, yes, that's a valid concern in general when talking about free speech: censorship is one of the most effective ways for authority to consolidate power). The most important reason is that we deal with crackpots by making them look like morons -- which usually takes no effort whatsoever. All you need to do is let them yap away. On rare occasions, a response is appropriate (i.e., exercizing your own freedom of speech).
Here's the rub: If this guy hadn't been arrested, he would have been completely ignored.
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* ETA: That is, Britney used that argument, and I think it's completely germane and valid, but it's not by any means the only one, and I think it's somewhat less important than other arguments in a case like this one.