*It is our intent that most of these devices will qualify, but some hardware/software requirements apply and feature availability may vary by device. Devices must be connected to the internet and have Windows Update enabled. ISP fees may apply. Windows 7 SP1 and Windows 8.1 Update required.
Some editions are excluded: Windows 7 Enterprise, Windows 8/8.1 Enterprise, and Windows RT/RT 8.1. Active Software Assurance customers in volume licensing have the benefit to upgrade to Windows 10 Enterprise outside of this offer. We will be sharing more information and additional offer terms in coming months.
So, it clearly mentions: Windows RT/RT 8.1 is one of the excluded editions.
About the free Windows 10 free upgrade offer, here are more details.