No problem. So the deal is that some schools just feel entitled to set their dates to April 15th, even if they don't start early (Baylor for example). Some that do actually have a good reason to ask early are ridiculously late with everything (*cough* UChicago *cough*), and then don't give you enough time to decide. Whatever, what they want is for you to narrow it down to one acceptance by this time. If you were waiting for second looks elsewhere, they might let you slide past the date. Baylor for example gave me until May 1st.
If you come off a waitlist somewhere else, feel free to go there. You are supposed to have 2 weeks to decide from the time you are offered a waitlist spot if you come off the waitlist somewhere else, even if you are only holding one acceptance at the time. The truth is you aren't completely locked in until you start a program. Nevertheless, the programs certainly would like you to think you are locked in earlier as it makes it easier for them. My roommate for example jumped out of UVa in the beginning of June to come here. UVa's first year class last year had one less student than usual. Oh well, one more they can take this year!