We obviously won't know for sure until FISA reaches the Senate floor, but two things spring to mind: Pragmatism, and political showmanship.
Pragmatism is the safe bet. Obama, Presidential nomination notwithstanding, is still the junior Senator from Illinois. It's true that he has momentum going for him... but that's not the same as political capital.
Bearing that in mind, it may be that he sees greater benefit to his election efforts by avoiding a fight now and biding his time until after the election. At that point he would be in a position to not only revisit FISA, but to appoint progressive (and Constitutionally-minded) Supreme Court justices who could potentially strike it down on Constitutional grounds. In any event, I for one am confident that it would be added to the laundry list of laws and executive orders that would be reviewed and likely overturned in the first year or so of an Obama Presidency.
But what if there's more to his statement on retroactive immunity?
...I will work in the Senate to remove this provision so that we can seek full accountability for past offenses.
(emphasis added)
Sure - maybe he means that he'll push for an amendment to remove amnesty. But aside from proposing an amendment, what could he do in the Senate to remove the immunity clause?
One word: filibuster.
How exciting would it be for Obama to actually stand on the Senate floor filibustering against legislation that epitomizes the excesses of the Bush administration? How inspiring would it be to see the person we hope to elect as President literally standing up and staring down the old guard? And how awesome if he chose as his reading material, say, the record of Congress from the evening of June 9th (Kucinich as read by Obama)? Not to mention the fact that it would undoubtedly get him hours of free television coverage.
I know, I know - it's probably just wishful thinking (especially that last bit). But as I've watched the debate rage here and elsewhere, I couldn't help but imagine what a fine example of political theatre such a spectacle might make...
...and Obama's nothing if not a savvy politician.