For some other vehicles (Volt) I've heard the leasing company gets the credit if you lease; probably ought to be the same for Tesla. Dunno why they wouldn't be able to pass that along.
The residual value guarantee is pretty good though.
because tesla doesn't lend money, they make and sell the cars, they do not own them once they are sold
usually there is a separate entity that finances or leases the cars sold at the dealership, such as mbfs for mercedes benz financial services, gmac for general motors (defunct) etc
for tesla to offer the rebate directly they would have to setup a revenue split from the finance back end, or take a hit on the sales revenue, which is either difficult to get out of a bank, and not in the interest of tesla, respectively.
The residual value guarantee is pretty good though.