“I agreed to support FWD, because there is a genuine need to reform immigration. However, this should not be done at the expense of other important causes. I have spent a lot of time fighting far larger lobbying organizations in DC and believe that the right way to win on a cause is to argue the merits of that cause. This statement may surprise some people, but my experience is that most (not all) politicians and their staffs want to do the right thing and eventually do.”
In short, Musk wasn't willing to do the same cynical DC two-step of John Doerr, for reasons of either principle or public image.
David Sacks of Yammer fame has also pulled his support alongside Musk. Both of their bios have been erased from the FWD.us online list of supporters, but it'll take a lot more than some removed JPGs to repair this for Zuck—this is a huge blow, from both a PR and money in the bank perspective. The blind goodwill we assume from tech benefactors starts to fade under even the slightest scrutiny, and will now only fade faster.