The situation with abusive relationships is not analogous to the joke because it is not obvious to people who ask, why there would be such barriers to leaving a relationship (i.e. why such an assumption would be unjustifiable). People who ask "why don't you leave" are typically not aware of the usual barriers, nor do they have any reason to be aware of those barriers.

Furthermore, the question often comes up in cases where one party did leave, but kept coming back. So no, I don't see how the joke is helpful or how it shows poor assumptions.