Think about it. In a world populated by telepaths, how could you ever be certain that your thoughts aren't constantly being read? Would you willingly set foot on such a world, knowing that? Has there ever been any indication (onscreen or in print) that this danger wouldn't be real? One would think - hope - that Betazoids would never read someone's mind without their consent, but has this been confirmed? I mean, look at Lwaxana Troi for example. She is friendly enough (maybe too much ) but I seem to remember a few times where she poked into people's minds unprovoked. Just ask Picard. I'm just guessing here that a native Betazoid can tell if another is reading their mind, but there's no indication that any other race (like a human) could. If one of us sets foot on that planet, a Betazoid could read your mind and you'd never know it.