It is
exactly a matter of spite. tl;dr: he is playing by the rules. people are spiteful because they think he is abusing a loophole in those rules (I disagree).
Saverin played by the rules: he legally became a US citizen. He got the related perks (access to education, quality of life, ...) and the related liabilities (The US tax code is onerous, and while it IS competitive with e.g. Europe on a percentage basis, you get much, much less for your taxes).
He is still playing by the rules: he is invoking the "I quit" card. Note that contrary to the media hype right now, he is making a bet: if Facebook loses value between his de-citizenship and the time he can sell (possibly 6 months after IPO, depending on how his stock was allocated and how the IPO is structured), he is going to LOSE money by denouncing his citizenship.
If he is not planning to live in the US, that's perfectly understandable - read about FBAR and FATCA. the tl;dr of those is: you have ridiculously onerous reporting requirements, FOREVER, with penalty 50% of your holding, PER YEAR (that is, if you're decided to be willingly noncompliant for two years, you have to pay your liquid worth; if you are for 4 years -- well, you owe twice as much; and by default you are assumed willingly noncompliant).
Now, whether or not his decision to move out of the US was driven by the US tax code or not, is (or at least, in my opinion, should be) immaterial. Furthermore, since he wants to live in Singapore, he is required to give up his US citizenship to receive a Singapore one.
I find support to this being a matter of spite, is that all proponents of this law that I have heard talk about punishing him and his ilk (in slightly different words).
Ted Arison and Michael Dingman did this in the early '90s and avoided billions in taxes (The wikipedia article claims Arison did pay taxes[citation needed], but I remember the headlines at the time said he didn't, and http://www.nytimes.com/1995/04/12/us/some-of-rich-find-a-pas... indicates that he probably didn't).
Saverin is actually taking a financial risk here, playing by the rules that were enacted, which are not letting him out easily.
It is exactly a matter of spite.
edit: minor rewording one minute after writing.