Hello,
Thanks for bringing this question to our attention! We recommend Defender AV on devices with Microsoft Team Room Standard and Premium, which is on by default. Most Teams Room MTR systems run Windows IoT and we have Microsoft Defender for IoT available. You can check out the details for that here:
https://azure.microsoft.com/en-us/services/iot-defender. For SMB scenarios with Microsoft 365 Business Premium and Microsoft Defender for Business (MDB), the device will onboard to Microsoft Defender for Business if you add it to Azure Active Directory; however, I don't have licensing guidance for this scenario yet. I have requested clarification and will respond to this thread once I have it.