First of all, there is no such person who judges not. But why some thinks otherwise? Why the twisting of Matthew 7? The motive comes from the unwilling desire for the SELF to be holy and righteous before God. Why not? It's not practical enough. It's not right to be self-humbling.
One need not be surprised, especially when those who love to tell others "NOT TO JUDGE OTHERS" change their doctrine (e.g. from calling homosexuality a sin to not-sin like Obama). They may not now, but they will change their doctrines.
Because when they say that "Thou shalt not judge", they do not want to listen. They do not wish to be taught by others. They would say...Only God can teach them, not people. Or they may not say that. Because bottom line, they care about what's pragmatic - that which is good for them. Some would struggle a bit, trying to live as of God would want them to, but in the end, when some twisted preacher come along, showing that them "Did God actually say that?" as the serpent did, they would falter because the desire that the serpent offers is far too practical not to ignore. They just needed a permission from someone that seems to have the authority, such as someone with a title of a minister. Some would say, if the minister preached wrongly, it's not the listener's fault...heck, maybe it's not even the preacher's fault, but God's fault, if the listeners were to falter. God's fault in allow such title to be carried by his enemies. Those who knew better, they know that they have run into their own folly. Then there may be some grace of God left in them, for them to repent.
Of course, the other reason, the traditional reason, when one says "Do not judge". But being that by saying that, one has already judged. They say it out of pride.
Conclusion:
The "gods" of these, though they may still be shown God's grace and thus repent, are pride and pragmatism. Thus they MUST always change their theological stands to keep their "gods" happy. They would not allow a 30 year old man who surrounds himself with sinners teach them anything, if that man just a mere carpenter. No, that would not be pragmatic enough. If it were to be a lesson from God, they would need it to be very officially made, publicly proven and accepted, heck, they would call God to display Himself in a supernatural manner, in order for them to confirm a lesson from God. Otherwise...JUDGE NOT, or face crucifixion!
Lesson:
Easy - This type of people is easy to identify: They love the material world (pragmatism), they may claim the love of God, they may even believe it. They would seek after preachers that preaches both mammon and God, to get approval from "god" in their desire. What they would not do: They would not truly be able to be Gospel centered. They cannot truly show love in reach those who need the Gospel. They may support, but cannot do it themselves. Not because God calls not them as they may wish to claim, but because they DO NOT WANT this calling.