Do all colleges follow the May 1st deadline for admission?

May 1st is the most common decision deadline, but not the only date used, and it usually only applies to regular admissions offers, so you should check with the schools that have admitted you to be sure that they don't have an earlier deadline for your specific offer type.

Why May 1st? Most colleges and universities in the US (but not all) belong to a trade organization called the National Association for College Admission Counseling (NACAC). As a condition of being a part of this organization, they agree that generally they'll give students until May 1st to decide. The idea is that if all of the participating colleges and universities agree to wait until at least May 1st, that gives students a fair amount of time in which to make a decision. There are exceptions to this guideline, of course. For example if you're doing early decision, or if you are an athlete on a sports scholarship, or in some other cases. That's why it is so critically important that if you have an acceptance from any colleges that you look at that school's specific policy for your type of offer, rather than just assuming that May 1st is the deadline.

Source: NACAC publishes its policies and statements at http://www.nacacnet.org/about/Governance/Policies/Pages/default.aspx and you can view their latest "Statement of Principles of Good Practice for 2015" in which they specifically reaffirm May 1st and their decision deadline guideline.