This is a statement of fact, not a admonishment or guideline on how to treat people.
We can accept this as Truth and still approach the unbelieving or fallen away with love and respect. Sometimes we forget that He most of all has the right to be offended by mans rejection, yet still chooses to offer Salvation. Can we hold ourselves in any higher regard?
I think Christ used that scripture to admonish one of his apostles. If I'm correct one of the apostles was complaining to Jesus that another person was seen casting out demons in Christ's name. Jesus said "he who is not against me, is with me".