They offer FL rates to fill the ship in a category that is not selling or that they can get a higher dollar for from new guests. For example, if you have a 5D booked they could offer a 5A category and a FL resident would have to move from their 5D to the 5A to get the discount. DCL chooses the cabin in most instances and so if you really liked that 5D you chose a year ago, you take your chances on which 5A you are going to get. Those 5D's get sold to new clients paying the current rate and the 5A's get filled by FL residents who might save and take their chances on what they get. AND, they are taking the chance that the cruise they want to go on is even offered at that special rate. So, as a FL resident I buy what I want on and choose the cabin I want and if a FL rate comes in with a category I am willing to switch to, then I take it. If not, I don't.
As a Disney shareholder, I want them to maximize their profit. If the goal is to fill this ship in a category that is not selling well, then why not offer the discount to a wider audience. Seems like they would be able to find people with offering a smaller discount. Disney is free to restrict this to Florida residents, but I'm not sure what advantage they gain by restricting it this way.