Because it's not equivalent. If I vote for Obama (and I won't), Romney has to attract two votes to win (one to cancel the vote for Obama and one to create a margin). If I don't vote for Obama, Romney only needs to attract one vote to create a margin.
If you and I both vote for Romney, Obama needs *three* other votes to win.
If I vote for Romney, and you abstain (or vote for a third party), Obama needs only *two* other votes to win.
Like it or not, if you don't vote for Romney, you do indeed help Obama win.