The RT does not need one. It gets the upgrade through the update process only. That product key is already in the system. The Product key is for regular desktops and laptops. For those machines who download the ISO image of the Preview, the Product key was posted on the web page just above the download links.
The RT is already covered for a Product key when the final version comes out and that will be through an upgrade process like you did with the 8.1 Preview.
You have to remember that the RT is in a world by itself and there will be no version of Windows that you can buy for it.
For those who have regular computers, they will have to buy the final version of Windows 8.1. The beta will probably expire in about a month or two after the final release is on the market. With the RT, you are already covered.
Additional note: if by any chance there is a charge for the 8.1 upgrade (final), then throw the tablet away and get something else. These tablets can only run what came with them and you are stuck with it. You cannot wipe them out and put Linux on them like you can with the Pro.
Personally, I don't think Microsoft will try to bilk people with the RT tablets. I have never heard of any issue regarding that.