The return of the feature means that the new buyers will be able to use Superchargers on long-distance travel free of charge. Something that the Tesla Model 3 buyers can't enjoy.
The Free Unlimited Supercharging was included in new Model S/X offer from the start for several years, but then Tesla removed it, using it from time to time as a promotion in the referral program or used Model S/X sales.
Elon Musk said in 2018 that the Free Unlimited Supercharging:
- is not really sustainable at volume production
- doesn’t incent optimal behavior
- We probably should have ended this earlier
The only reason for re-introducing the Free Unlimited Supercharging that comes to mind is decreasing sales of Model S and Model X. That adds to the significant upgrade (Raven) and lowered prices, which probably still wasn't enough to improve declining sales results.
Tesla probably needs to increase Model S/X sales until at least the launch of the Model Y, which combined with the Model 3, could bring enough volume and revenues to maybe finally provide for significantly upgrading the Model S/X.
As of now, Model S/X batteries accept up to 200 kW of charging power (compared to up to 250 kW in the Model 3). Supercharging will be slower but free.
According to Electrek, the current edition of Free Unlimited Supercharging is available only for the time of ownership, so the next owner will need to pay as he/she goes.
"Electrek saw a memo that Tesla sent to the sales staff today confirming the change and noted that the ‘Free Unlimited Supercharging’ is for the time of ownership of the vehicles and is not transferable."