I've only seen this sort of thing from automakers of limited production supercars. It's meant to keep the very wealthy inside their formal or informal buyers programmes, where members are expected to spend a certain amount per year or purchase less desirable models in order to get on the lists for the good stuff.
That certainly doesn't apply here. This is a mass market car by a mass market car maker. So we can only conclude that Tesla suspects either that resale values will be well under their asking price, the demand for the truck will be limited enough that used sales will directly affect new car sales, or both.