Roger Williams was also exiled from the other Puritans in Massachusetts (5 years after arriving in Boston). Not implying that was related to his colorful usage of the English Language, but marginalizing the influence he had on the Puritan culture of the New England region. Can't really say he represents all Puritans and is just one example of those that ended up settling in Rhode Island. He also became a Calvanist/Baptist following his exile to Rhode Island after disagreements with the Puritans in Massachusetts, so he was not exactly a Puritan for long after settling in the New World. I know I'm reaching with this statement, but I would say he probably had a slight contempt for what was considered "normal" or "acceptable" by others at the time as well so cursing would fit his personality.
But more to your argument, Roger Williams was a Puritan, but anything but a prude. He figured that going to Hell was punishment enough to justify practical acceptance of non-believers in this world.
Our National myths don't necessarily conform with the facts, amigo.