This explains it in better detail than I could. It isn't so much a single law, but rather a lot of different laws that all work together to ensure that Israel is given everything it could possibly need for its own defense...even bypassing normal regulatory measures in the process, as long as certain conditions are maintained.
Israel has invested tens of millions of dollars lobbying for, and helping to write, the legislation that guarantees their continuous access to weapons. A lot of it is written directly into defense spending bills that are essential for domestic defense spending as well. So, without a massive shift in policy at the legislative level to disentangle those priorities, that support is legally binding.
Gee. It's almost like they had advanced warning that things in the middle East were about to go sideways. But, sure...tell us again how Netanyahu went rogue by attacking Iran. It makes so much more sense to believe this was an unexpected turn of events, and the US knew nothing of his plans. /s