The model only works if users are forced to subscribe to a battery swapping service for the full life of the vehicle (or there is a large upfront fee to join with a used vehicle). Otherwise it would be too easy for a consumer with a worn out battery to do a one-time swap and get a like-new battery as a cheap alternative to very costly battery repairs. The dumped battery is likely to have very poor range and the battery swap company will need to dispose of it.
That is why they make you lease the battery. You cannot swap out your old battery, just the battery you are leasing. Your lease payments include the cost of them replacing batteries.