There is a very good reason: a humanitarian one. I do not know how a nation where 70% of the population calls itself christian could turn away refugees from war-torn countries.
> there is no reason for non diplomatic personal Iranians to enter US at all.
Top US universities and tech firms beg to differ. The US got the best universities and leading tech firms exactly because of immigrants. Paul Graham's essay about this is interesting: http://paulgraham.com/95.html