Why is getting your PhD in Iran invalid for obtaining a post-doc position in the US?
I think the answer is simple: your premise is false. There are no rules/regulations forbidding an Iranian earned PhD from being a postdoc in the US.

It might be hard to get a postdoc in the US for anyone whose academic background, and PhD earning institution, is not perceived as highly ranked.  



Note: I have answered this a year and a half ago. Things may have changed since then through the presidential ban against Iranians entering the US. I have no specific knowledge of the current situation with respect to Iranian students.