Is not extending the Islamic greeting (Salam) to Muslims who do not adhere to the verbal and practical Sunnah considered a sin?

A Muslim's greeting of peace (salaam) to another Muslim he meets is a Sunnah, even if they are not observant, unless shunning them serves as a deterrent from a forbidden act they are committing. Returning the greeting is obligatory. There is no sin in not initiating the greeting, because it is not obligatory. However, whoever intentionally abandons it has forsaken a Sunnah of the greeting of Islam.