We're not talking about head coaches, we're talking about assistant coaches. It's pretty much understood that head coaches will leave at the drop of a hat - that's why buyout clauses are inserted into their contracts. Also, head coaches are given multi-year contracts, whereas the standard for assistants is to be on a year-to-year basis. The time for coaches (assistants especially) to change jobs is the period between the end of the season and signing day, because that's when their annual contracts expire and their freedom of movement is not restricted. Think about it - when was the last time you heard of an assistant coach leaving for a lateral position 3 weeks after signing day?
For example, Coach Kill's Minnesota contract included money from the U to pay NIU for his buyout. Again, this is pretty standard. If NIU wanted, it could've toed the line and held his assistants to their contracts until 2/1 (or whenever they expired), but given that the new head coach would likely hire his own assistants anyway, it's pretty much a no-brainer for the university to release employees from their contracts without penalty when they would likely leave (either voluntarily or otherwise) as soon as their contract was up.
I'm not 100% certain on the rules re: assistant coach movement (I may be wrong - I have been before), but they are certainly not the same as the rules re: head coach movement.