I can hear you all screaming now. Guac should never have either of those wretched ingredients! Ugh, what are you thinking!? However, innumerable recipes (even Emeril's) call for one or the other. And I worked in a Mexican join that used sourcream. Folks raved about the creaminess and tang. And most of the US doesn't have easy access to ripe avocados, and the ones they get are expensive.
So really. Just how irate would you be if you liked a guacamole that you were served at a restaurant and later found it had one of the above ingredients? I'm not talking vegans who know to ask their server.