I there I can clear this up for you somewhat... A similar thing happened to us, we rented a van to go to Pennsylvania a couple of years back. We had no problems at all, and in fact thought hey instead of putting the miles and where and tear on our van why not rent one.. It was cheap enough...
when we told them we wanted to go FLorida in it they said no way... For insurance reasons was the excuse... they wanted us to get insurance per day which was fine except the 14 day rental was going to cost us $ 1200.00
$ 1200.00 for two weeks of insurance and rental.. We flew for half of that and rented from Alamo when we got there... And saved oursleves $ 400.00
Also our own insurance agent said it is because of liablity that they don't want the cars to go down that far, for example if you get a speeding ticket and the car is a rental, that particular state can come after the rental agency if you don't pay that ticket once you return !
... I can suggest renting from across the border, and take it down...
Good Luck