Is there an informal understanding at universities about PhD student funding? This question was prompted by answers and comments on this question about PhD student funding.

I am under the impression that STEM Ph.D. graduate students, when accepted into an RU/VH university in the United States, will typically be funded for at least 5-6 years, depending on the average length of Ph.D. in the specific field, by either TAs, RAs, a mix, fellowships, etc.

I understand that my university, the University of Arizona, has shirked this duty in a legally binding sense, but is this impression wrong?

If it is, why would I (or any other graduate student) risk having to find a job in the middle of a Ph.D. in order to "pay the bills?"
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.