A bjj player will view escapes from bottom as 'taboo' or uncultured in training because the rules actually reward you for staying down; standing up gives your opponent an opportunity to score on you, while sitting down yourself puts you in the same situation, but takes away the scoring opportunity. In fact, standing up is also giving your opponent the opportunity to sit down, getting to a 'safe place' (in terms of scoring opportunities) without giving you any points like if he had done it while you were down (even if the ending positions are identical).
From a gamesmanship perspective, there's little to no benefit to spending training time on working to escape from a bad situation (being on bottom), because you are not formally penalized for being there in the first place (except in the most absolute terms of biodynamics); indeed, it's actually an advantage.