There are cases where a rabbi is forced to guide someone to the least of evils. If they announce the ruling, or even aren't careful as framing it situationally -- something that must be decided on a case-by-case manner -- they could be lowering the communal norms.
Here are two contemporary examples. I should point out that neither are necessarily popular rulings. (Or maybe they are, I didn't take statistics.) Just that they are actual rulings by Orthodox rabbis.
The rabbi quietly tells the single woman that given that you told me you will have premarital sex regardless, it is better you go to the miqvah after being a niddah. But he should not publish this as a responsum or newspaper Torah column. Because then women who would have otherwise abstained (or abstained more often) would now think that premarital sex after miqvah "isn't so bad" and there will be more sin overall.
Or, the outreach rabbi is told by his Rosh Yeshiva that it's okay to invite someone to come for Shabbos dinner or davening knowing they'll probably drive. In the situation that it's the only way they'll have Shabbos at all, they'll be driving somewhere anyway, and sometimes it's okay to violate one Shabbos if it's the only way to observe future Shabbasos. But not announce it as the movement's policy. (As did the Conservative movement in 1950.) Again, making a least-of-evils allowance can't be presented to the masses who will mistake it for the message that the lesser violation isn't so bad, or perhaps isn't really prohibited.