I suspect that Tesla is treating this $5k refund as a rebate, which doesn't include refunding the tax, to make things a lot less complicated.
If they were to do it as a real refund they would have to update the MVPA which would likely require involving the financing company, if any, and could even require getting a new loan at now current rates. At a minimum I would imagine that the financing company would require that the majority of it be sent to them, as otherwise it could be seen as refunding your down-payment.