Allowing Iran to benefit as a member of the world community incentivizes their cooperation with Western powers. They may believe deeply in Sharia law and govern through religion, but disrespecting and persecuting those beliefs and systems only reinforces their need to seek alternate means of gaining power in the global community (i.e. insurgency and nuclear arms). In effect we are pushing Iran to fund terrorism and pursue nuclear weapons because it is their only chance at having any chips to negotiate with. It may be true that they aren't interested in negotiating away their nuclear arms or connections to insurgent groups, but they have no reason to trust us when we also are the biggest backers of Israel (a nation that does have nuclear weapons and pushes even harder to persecute Iran). In the we have two options to maintain control, make a deal with Iran that meets both our needs, or completely subvert their government in favor of one more friendly to the west, but just putting pressure on them like this makes them more dangerous than ever.