The Gemara sometimes relates anecdotes of Amoraim and presents them in a negative light by including details which would seemingly classify as Lashon Hara in any other context. For instance,
"רַב אַוְיָא אִיקְּלַע לְבֵי רָבָא. הֲוָה מְאִיסָן (בֵּי) כַּרְעֵיהּ בְּטִינָא. אִתֵּיב אַפּוּרְיָא קַמֵּיהּ דְּרָבָא. אִיקְּפַד רָבָא, בְּעָא לְצַעוֹרֵיהּ"
'Rav Avya went to Rava’s house. His feet were dirty with clay and he put them on the bed before Rava. Rava got angry and sought to torment him with questions.' (Shabbat 46a)
Why would Rav Ashi and Ravina name the sage and also include these embarrassing details? Why would the laws of derogatory speech not apply?