Why can't they refuse?
Lets look at this time and place.
The US is looking to get a UN resolution regarding Iran's nuclear ambitions.
If Israel refused to obey a UN resolution, the US would be forced to impose sanctions on Israel or render any such resolutions empty of purpose and meaning and they become an empty and useless gesture towards Iran.
Without US aid, Israel would be hard pressed to continue. However, with Iranian aid, Hizbollah will fail. The resolution is binding and mandates that aid end, but it doesn't end our aid. So again, Hizbollah and Iran lost the cover they previously had. Now, the US and Israel must work in tandem to stop Syria, Iran, and its proxies, Hizbollah.