Because if they have one now it is only for its defensive reasons, so the West and Israel does not attack it, just like they attacked Iraq and Libya. Iran should reserve all its right for any weapons for defensive purpose. I believe whether they have it or not the West will still bombard them. One has to look at the recent history. Do not give up your weapons Iran.
Syrian troops sieze village above rebel-held parts of Aleppo (Reuters)
from the article: UN report on Iran nuclear program a fabrication, says FM