In simple terms, property ownership and residency are two entirely different things.
There are plenty of Florida residents who don't own any property. They're called renters.
There are plenty of Florida property owners -- whether of vacation homes, land, office buildings, or timeshares -- who aren't Florida residents because they live in another state.
As Dean correctly points out, Disney could offer Florida resident discounts to DVC members anyway. It would simply be a matter of calling the program "Discounts for Florida Residents and Disney Vacation Club Members." But, so far, Disney has not done so.
If Disney ever determines that they could make more money by extending Florida Resident discounts to DVC members, they'll do so. Until then, we'll just get the limited DVC discounts.
My guess is that Disney would come out ahead financially if they allowed DVC members to use Florida resident discounts on annual passes and allowed DVC members to buy memberships in the Disney Dining Experience (currently only for Florida Residents). I believe I'd end up giving Disney more of my money. It would push me into buying annual passes and going to the parks and Disney restaurants every day that we're staying at a DVC resort. It's in Disney's interest that as many DVC members buy annual passes as possible. (But currently, Disney only seems to be looking at "lost revenue" from those DVC members who would buy annual passes anyway.)