Mystery solved! Probably! Thanks our perceptive readers, we have learned what was maybe on the mind of the lady we posted about yesterday, the one who was gently blocking access to an empty seat. Also, we've learned that SFist readers are violent, flatulent racists.
Here's what she was probably thinking: "In my country of origin, I learned that public transit is lousy and over-crowded. So if I want to get off of this rumbling deathtrap someday, I must be forever perched, cat-like, by the exit." Or something to that effect.
Thanks to tomate4369 for pointing out a cultural aspect to this seat-saving strategy that simply hadn't occurred to us; and thanks to our other commenters who pointed out that special accommodations may be required by the very tall, those with leg injuries, and the very demure ladies who wish to avoid a potential groping.
Also, to the bilious individuals who gleefully report banging seat-savers in the head with purses and subjecting them to digestive winds: consider practicing some anger-management, and consulting a gastroenterologist.