Chancellor Wise of UIUC, and the board of trustees as well by implication, has said that the (de)hiring of Salaita was due to the manner in which he expressed himself, not the content of his political views. Clearly the university cannot say that it is the content of his speech, since that is protected under the 1st amendment and academic freedom. So the argument has to be that it is the way in which the message was framed. It is a violation of tone, or etiquette.
I think that Salaita would not have been de-hired if he had tweeted in a different tone, or avoided skating on the the edges of anti-Semitic discourse in an ambiguous way. After all, a lot of the existing faculty members at Illinois are pro-Palestinian.
Here's where it gets a little tricky. Universities cannot discriminate against viewpoints (theoretically). But they discriminate against people either, based on religion, race, et... So a willingness to skate on the edges of hate speech would make someone a difficult hire for any university. Of course, UIUC has employed a racist (allegedly). I think they could not have legally fired him, even though I would not trust him to treat non-white students fairly. If this individual were hired by a department and the chancellor got wind of his views before the board of trustees voted, would she intervene?
Racism, as a political view, is protected, but discrimination based on race is illegal.
So, in this sense, the decision to de-hire Salaita does depend on a parsing of his tweets for both tone and content. If the university lawyers can argue that his tweets are hate speech, and thus that he is likely to discriminate against students of a particular religion, then they might get a sympathetic hearing. The problem here: I myself, who abhor Israeli policy, saw his tweets as anti-Semitic. (I am anti Israeli and philo-Semitic.) I had to read and argue with others over the course of many days to even see why the other interpretation, the intended one, was likely. I got impatient with people who said that the intended meaning was somehow obvious. Since I think I'm pretty smart, you can see where equally and less smart individuals, or those motivated by malice, could, like me, need a lot of convincing to see that they are wrong.
The central analogy justifying Salaita's hire, and a lot of his own work. is that Palestinians and Indians are both indigenous in a parallel way, both the victims of "settler colonialism" in their respective situations. This analogy breaks down at some key points. For example, some Jews are also indigenous to Palestine. Others, Sephardic or Arab Jews, were kicked out of Arab countries (or left voluntarily, seeing the writing on the wall) after the founding of Israel. They are indigenous to Med. region, whether Morocco or Turkey, but not to Palestine / Israel, except by their distant ancestry of course.
We can see this as a reshuffling of populations in the wake of the dissolution of the Ottoman empire and the end of WWII. How does this compare to European colonization of the New World? Not very exactly. Where does this tie to classic discourses of anti-Semitism? The idea that Jews are not indigenous to anywhere, that they are rootless and cosmopolitan. Of course, this rootlessness stems from the fact that they were kicked out of everywhere. A Syrian Jew living in New York could probably trace her ancestry to Spain.
My final point here is that Anti-Semitism is a real thing. It's not just some cloak exploited by the Israeli govt to exempt itself from criticism (though it is also that). The emancipation of Jews happened relatively late in European history. People like Martin Luther were violently anti-Semitic, and this was not even seen as controversial until after the Holocaust. We're still trying to get a Facebook page on Jewish ritual murder taken down, under their own policy about hate speech.