WinnieThePooh,
Once against “tznius” is not the right word to use here. A Rosh Yeshiva, if he knew what those places are, would not tell a talmud it was mutar because the whole purpose of such places is schok, which is asur to engage in with the girl.
You might be right that he would also tell him that it is not the right environment for a Yeshiva boy altogether (although that is a “grey area”), but from the fact that so many Yeshiva boys, from top yeshivos, take girls there on dates, tells you that either the Rosh Yeshiva don’t know what these places are, don’t know that their boys are going there, or are looking the other way because of “mutav sheyiyu shogagim.”