JWs who are loyal to their God will not attend the worship service of other Christian faiths....?

Because it is incredibly smug and arrogant. There is a base level of human respect one should give to all other humans and their religions. To not practice another religion simultaneously is fine, but to not even walk into their buioldings and observe them worshipping is a giant insult. To consider all other religions "whores" like JWs do, thaT ALSO is incredibly smug and hateful. It goes beyond just not practicing with them, it hails insults with injuries.