Rand probably isn't a fan of the U.N., so he wouldn't much care for the idea of a U.N. dispute process over any event really.
He probably also thinks that the Palestinian/Israeli issue is really none of our business and we should actually step back and let them sort it out.
I could see this letter passing Rand's desk, and him skimming and signing it.
Rand's platform plank to cut foreign aid (including to Israel) and support or lack of support for Israel most likely had nothing to do with his decision.
After reading the letter, I have no problem with him signing that letter, at least from an ideological point of view.