I thought the USA already made it clear that they won't help Israel if they're the aggressor.
If Israel attacks and Iran does nothing for awhile then it's a separate incident.
Later if Iran does something offensive to Israel it can be seen that Iran is the agressor even though it's retaliation for a past incident.
The US doesn't want Israel to attack and may say things to dissuade Israel from doing so. Saying they won't help is one way of doing this. I don't believe what they say and that's my concern, the US can change it's stance whenever it suits them.
I don't want the US armed forces in any more danger than they are already.