I would have to imagine the rule is in place to help facilitate social distancing for the sake of guests staying at those resorts. Think about resorts like the Poly or CR that frequently have non-guests visiting for shopping, to look around. Not to mention guests of other resorts who use these spaces as a rest break during park days to avoid eating up time going back to their own resorts much further away. If only people with dining reservations are allowed in, it cuts down a lot on the number of people packing lobbies and common areas and keeps them open for people actually staying there.
I honestly think it's one reason 'Ohana isn't open initially, besides the fact that it's a character meal by day and churrascaria by night. That place is a magnet for people coming from the outside and they're always crowding in around Tambu and the lower level of the lobby.