E-G-O
And KC DOES have the cap room to keep Trent, which would be a horrible move, but Carl Peterson is just that full of himself.
A KC Radio guy made a great point this week.....How many times do you strike gold in the 5th or 6th rounds anyways? That is what they are fighting over. For every 1 Terrell Davis or Tom Brady there are 500 R-Kal Truluck's and Jimmy Wilkerson's.
This is all about the vast ego's of two men, Mueller and Peterson. Both are despicable in my eyes.

This is the key reason why Peterson is holding firm. Having Green at this point is a luxury (what if Huard goes down in MC/TC?) while the lack of a legimate starter is a liability for Miami. I think KC has more leverage than most people realize.
Green's current market value in terms of draft picks/veteran player(s) is stable. And as injuries mount and as coaches lose confidence with their rostered QB's (Miami included), his value can only increase. If this happens, the number of trading partners will likely increase. Lack of supply increases demand which creates more competion for the player which results in a stronger bargaining position. And like you said, a 6th round draft pick is practically worthless. Why wouldn't Peterson hold out for more?
"The King" is only doing what you or I would do given the same circumstances. In fact, we do this sort of thing all the time with our FF trades (take advantage of our trading partner's lack of depth at a particular position). I normally don't side w/CP, but I like what he's doing here.