Now that you've taken the sensible, logical approach, look at it from the corporate angle.
Verizon has partnerships with these companies to offer hot devices that will sell, and likely in order to get the first of a kind, or the 'upgraded specs over the competition' model, they promise things like exclusive launch windows. Or even better, Motorola has a little cash in the deal thrown at Verizon to be the first with certain technologies. Of course, there are contractual terms such that if Motorola is severely delayed (like they are with the Bionic) they lose the exclusivity option. The carriers do have all the power, but they're leveraging that into additional revenue under binding terms. Now, the other side of the equation is the Verizon buys/orders X thousand units to stock so they need to move those units. There are bean counters out there that know how many folks are up for renewal and what the targets are for probable phone sales per month at all levels (midrange, high end, dumbphone, etc). What they least likely can predict is who will buy high end unit X or high end unit Y, so they limit to one choice per release period in order to move those units. Business is complicated, eh?