I think it is a built in survival trait; however, I would like to take it one step further & say that they LEARN over time that this look keeps them out of trouble. Mine have perfected "the look" and use it on unsuspecting guests for attention. Callie, in particular, has the most woebegone expression, and she uses it on those she can tell are suckers. A friend recently stayed with me for the weekend. All weekend, she was saying "Oh poor Callie! Look at her face." I kept insisting to her that Callie was playing her for a sucker, and she was feeding into it. She was convinced Callie was sad or upset. While we were watching a movie, Callie fell asleep on top of my friend on the couch. Movie over, I headed off to bed & she said she was also going to be going to bed soon. The next morning I found my friend on the couch with BOTH basenjis on top of her. Once Callie settled in, she felt like it would be mean to wake her up, so she slept on the couch all night so as not to disturb Callie.
I think it is instinctual/survival, but I think they learn to work it to their advantage for sure!