It is not the signing of a pro-Iran support letter; its saying you are a rabbi to give weight to your signature that matters.
In a post by Eric Golub for CommDigiNews:
LOUISVILLE, August 24, 2015 — The Jewish community has been buzzing over the news that 340 rabbis recently signed a letter in support of President Obama’s Iran deal. A deeper look into these 340 individuals shows that only 151 of them are truly doing the work that a rabbi does, while 189 of them are using the title of rabbi to convey a prestige on themselves that is not merited.
A complete list of those who signed the letter of support, and their occupations – many of which are excellent pursuits that are positive and good for the community – can be found here.
This list shows that the left’s claim that 340 U.S. based rabbis support the Iran nuclear deal is false.
To allow these 189 individuals to say they are rabbis is akin to referring to Jesse Jackson and Al Sharpton as reverends or comedian Father Guido Sarducci as a priest.