By banning all religions the defacto position would be Atheism which would make it a competing "religion". That would make it no different than forcing your own ideology on others in the same way of religions doing forced conversions.

Iran has crosses and kippa's by the way.

Even Stalin's crackdown of religion is often defended from the angle of destroying the power of the Church. But religious institutions already don't hold much significant power these days so you'd just be destroying people's cultures.