I am not sure it is "hidden" as much as poorly defined.

I can see the point as the closer one cancels to their rental time the less likely it is they can turn around and rent it. Yet I would still want to be able to know ahead of time I would be charged $X if I had to cancel.

Interestingly the only info I could find about their change policy is here: http://www.yelp.com/topic/san-francisco-i-hate-zipcar