Which principle?

Obama should not put the principle of public campaign finance ahead of the necessity of evicting the Republicans from the White House. Restoring good government and hopefully the Constitution has to come before the current ineffectual regime of public financing.

John McCain has already played the system, as Republicans so often do. When he was down, he borrowed money, essentially even if not contractually against the possibility that he could pay it back with federal funding even if he himself could not raise the money. When the private funds started to roll in again, he declined the public funding. Whether this is legal is a good question, but the toothless FEC will do nothing timely or significant to punish it.

Further, the settled race in the GOP would permit McCain to spend his primary funds to fluff his own reputation, while Obama and Clinton would have to continue their primary spending against each other for as long as the race remains in doubt - possibly all the way to the convention. The head start this would give McCain would be invaluable.

Democrats and Republicans alike have opted out of the system before. I'm sure that they will again.

Obama's going to need every advantage he can get. Giving one away at this stage would be stupid politics.