I think their parks really cater to different crowds than SDC. Most of their parks are just regional parks. SDC is a mid-level destination park. (On the same level as a Busch Gardens or Sea Wkrld but not a Disney or Universal)
Not to argue with you (well, ok, maybe a little. hehe) but SDC is much more a regional park than many of the Cedar Fair parks are. Knott's Berry Farm, Cedar Point, and King's Island all bring in visitors from around the world (and all 3 have more than 3 times the attendance of SDC), whereas SDC really draws from 4 states- Missouri, Arkansas, Oklahoma, and to a lesser extent Texas. While we might like to think it's a destination park, it really isn't. It's crowds are very much regional, and the vast majority are from within 2 hours. It's numbers and drawing power are nowhere near what many of the Cedar Fair parks are. And then there is the fact that they only have one park that has any sort of overlapping sphere of influence with SDC (Worlds of Fun).
So really, SDC is a mere blip in Cedar Fair's eyes and nothing they are going to concern themselves with when it comes to competition for their announcements. For them they really wouldn't care if SDC is making the announcement the same day. They're dealing with a much larger audience with a much broader reach than SDC does.
Edit- or to put it another way, there are reasons that SDC plans it's operating schedules around local school districts' calendars and the Cedar Fair parks generally don't.