The US is declaring support for Israel, yes, but that is not merely a matter of Hamas et al hitting the wrong targets. By comparison, the US invaded Iraq because they had WMDs, which was a lie, and were behind 9/11, which was a lie, and because it'd make things better for people in Iraq, which was a lie.
That would mean that someone has followed Seanchaidh's suggestion and provided Hamas with modern weapons, instead of them building their own in their backyards.
Palestinian insurgents have very little influence on the US's domestic politics or religion. US forces aren't casualties in the conflict to any large degree, and while it's costing the US taxpayer a lot, that money is going into the pockets of the US military/industrial complex, who would rather things continue as they are.