How many other " Christian " based religions institute a rule like that?
The WTBTS will try to justify their shunning policy based on Matthew 18:
“If your brother sins [against you], go and tell him his fault between you and him alone. If he listens to you, you have won over your brother. If he does not listen, take one or two others along with you, so that ‘every fact may be established on the testimony of two or three witnesses.’ If he refuses to listen to them, tell the church. If he refuses to listen even to the church, then treat him as you would a Gentile or a tax collector."
So WWJD? He ate with tax collectors such asZacheus, He recruited Matthew the tax collector to be one of The Twelve! He associated with gentiles, including an oppressor of the Jews: the Roman centurion!
If you expect to convert somebody, you must engage them. Not partly, not halfheartedly, but fully engaged. You must truly be friend and family, not an adversary.