1) They can...
2) It's politics
3) Israel isn't considered a direct threat by the US Government.
because:
a) Israel is a Democracy.
b) Israel is a very strong and consistent ally of the USA.
c) USA has vest interest in Israel because it is the USA's strongest, and really, only ally in the Middle East.
d) The US needs Israel to reinforce it's own political agenda in the Mideast, that being, the Democratization of the middle east -- this is really what it all comes down to.