You see... it's different for each state...
Here's one I found for Michigan, and it's stupid complicated, and completely unfair (if you take delivery in Michigan they will charge you Michigan sales tax no matter where you are from, but if you live in one of the non-reciprocal states, that tax payment does not count, and you would be required to pay your own state's sales/use tax at registration). But a dealer in that state does not have a choice.
http://msdra.com/out_of_state_buyersales_tax.htm
Here's the one for Iowa: Note "Iowa collects a 5% state sales tax rate as a ""One-Time Registration Fee"" on the purchase of all vehicles."
https://www.salestaxhandbook.com/iowa/sales-tax-vehicles
How your state handles the Iowa collection of the tax or "one time registration fee" is up to your state. As far as I can see they are REQUIRED to collect the 5% tax (and Iowa calls it a registration fee, so it's not the dealer being nefarious).