Is there an informal understanding at universities about PhD student funding?
I cannot speak for all US universities or all disciplines. That said, the graduate programs in the natural sciences that I have been involved with  do knowingly commit in legally binding fashion to provide five years of support with the caveat that this may come in the form of 20 hour per week TA appointments if no RA funds are available from the PI or the department.  

Why would someone accept a lesser deal? If one has a better option, I see little reason to do so. If one does not, one might perhaps accept such an arrangement in order to receive a PhD education that, in many fields, vastly increases one's earning potential.