I assume one of the problems with DC paying a licensing fee to use the characters stems from how much profit the Milestone characters stand to make.
If, for example, DC wants to drop Icon in the Justice League, then, depending on the details of the agreement, the Milestone owners (McDuffie's estate I assume) would get a check. Same goes for a Static monthly or whatever. And how much profit potential do these guys have? Static had a cartoon and is sort of popular. He certainly has his following among comic fans. But could he make enough, long term and short term, to equal the price DC would have to pay to use him? And if the fees involved cut too deep into DC's profit margin, or the character doesnt pull that much money to begin with, it becomes a loss from the business end of things.