We do not know the extent of Rav and Shmuel's le'Shem Shamayim when they ate. The Pasuk "be'Chol Derachcha De'eihu" (Know Hash-m in all that you do) does after all, apply to such mundane activities as eating. The saying goes that 'Tzadikim eat in order to recite a B'rachah' (rather than the other way round, like most of us do).

Besides that, Shmuel might have been referring to Shabbos or Yom-Tov, where the Mitzvah of Oneg Shabbos or Simchas Yom-Tov applies. Or to a case where they were guests at somebody's table, and the host brought in the dessert (where it would nhave been impolite to refuse).

In all of these cases, without Shmuel's explanation, eating may otherwise have been a matter of Achilah Gasah (eating when one is full), which is forbidden.

be'Virchas Kol Tuv

Eliezer Chrysler.