It seems as though these boards exist so that people have an opportunity to see if they can put forth the one exception they believe disproves the rule. So charging only offsite guests for something all that used to be equally provided to all guests, regardless of accommodations, is a viable business strategy because some people eat PBJ while others eat steak.
Charging for FP+ isn't unfair and it isn't morally wrong. It's just problematic from a business perspective. Why? For one, those who stay on property now are already incentivized to do so, and have been with what has been, up to this point, equal treatment of FP.
Would FP+ provide an incentive for those who have demonstrated a desire to stay off property to move on property? Would this number be large enough to offset the number that would choose not to come at all? Would those moving on property adjust the amount of time they visit, the frequency of visit and the total amount of money spent?
We have a paradox on this board. Some people lament the low number of FPs available via prior booking while others believe current off property guests would pay more to access them. And greedy Disney, who hasn't charged for FPs up to this point, won't be able to resist now. My guess is, based on Disney's historical treatment of FP, is that the value is insufficient to incentivize behavioral changes across the target customer base to positively impact revenue.