Obama is turning over the decision to Congress. He understands that the vast majority of Americans are against the attack. Polls matter. Obama wants to be loved. He can push the responsibility to Congress and no matter what it decides, he can claim a personal victory. Obama votes present.
And when Iran announces it has the bomb, he can pull out this speech, change Syria to Iran, rinse and repeat. Yada, Yada, Yada....
I see it differentlty. David Cameron wanted to go to war and was humiliated. Obama will be humiliated in the same way hopefully.
This is also a huge, humiliating climb down for Obama. He was hot to attack and found little support, so now has to wait until Congress returns.