It may also be not that they think that they are talking about a unwelcome subject, but only that they recognize that not every conversation needs to be held publicly and recorded for posterity. If they want to talk about the weather, they should not do it in a thread -- not because it will be downvoted, but because it is rather rude to broadcast every conversation when we have a number of perfectly acceptable ways to hold conversations without distracting the entire site.

Of course, if "negative karma" was what they were really worried about, and this is not just your interpretation, it may be useful to hold conversations out in the open. At best you will be happily surprised, and at worst you will have am audience to encourage you to do your best when talking about questionable subjects.