Hamas' motivation for entering into the unity agreement is largely to wedge between the Palestinian Authority and Israel, who have reached a stable, if stalled, status quo. Fatah is motivated to reconcile in order to demonstrate its legitimacy and strength, especially given that it was forcibly run out of the Gaza strip back in 2007. The unity agreement gives Fatah inroads to Gaza and demonstrates that it can effectively bring about Palestinian unity.
Statements from Israel and the US are largely consistent with their policies. Israel says Palestinian Prime Minister Mahmoud Abbas must choose between peace with Israel or peace with Hamas. The US welcomed reconciliation efforts which promote peace, but reiterated that Hamas is a terrorist group.
Once the rhetorical dust settles, however, the US and Israel should seriously consider their position in the new status quo. Hamas has made a decision to re-enter Palestinian unity politics. Every objective they can accomplish through politics is an objective they will not try to accomplish through costly violence. The threat of violence will be ever-present with Hamas, but threats are much more manageable than a rocket landing in someone's living room.
While clearly steps to legitimize Hamas' use of violence and brutality against innocent civilians are ill-concieved, the focus of US and Israeli policy should be Israeli security, even if it means engaging with entities they rightly find reprehensible. The more political Hamas becomes, the more constraints it will have on the use of force and the pursuit of radical policies (see Sadrists, The). These constraints include internal political fracturing, stalling based on inexperience, and the political cost of violating agreements with Fatah and other entities.