I think it’s more of a capacity thing. They have to factor in the FFP sales in the nightly attendance cap, so now if they know X amount of FFP users won’t be showing up until later in the run then they can allow more single night ticket sales on those nights.
However, there currently doesn’t appear to be any incentive to pick a date later than Sept. 1st as your pass start date… but like others suggested, maybe this is the first step so that in the future they can offer a variable price pass based on the pass start date/number of nights.
EDIT to add - on the flip side of this, it’ll also be interesting to see what happens when nights sell out. I seem to recall them pausing the sale of FFP last year when opening weekend sold out… I’m assuming now when a night sells out you’ll no longer be able to select that night as your pass start date?