It's like this. AMCAS sets certain guidelines for medical schools as to how they should run their admissions procedures. Med Schools are expected to abide by them, but AMCAS is not going to take action against medical schools (especially good ones) since schools could just as easily pull out of AMCAS and get all their applicants without them.
The AMCAS guidelines say that you have until May 15th to narrow down all of your acceptances to one. Schools that are requesting an earlier response may have a good reason to be doing what they are doing, and they may not. UChicago starts kind of early for example, so they have a legit reason to ask for a decision by April (and yet they interview really late... grr...). In the long run, MSTPs are throwing around alot of money (most of which their own), and so some tell you to give them an early decision.
So, if you are waiting to hear from certain schools like UCSF/Harvard/Penn/Other Late Schools, feel free to tell the program you are waiting for them. They should be alright with that. If they are not, I don't think you should feel comfortable going there, but you would have to evaluate the situation then. Unfortunately, they are holding all the marbles, but it is still common courtesy to let you take your time if you have a good reason.