Building a UHF RFID Reader, FCC regulations?
I recommend going over TI's technote ISM-Band and Short Range Device Regulatory Compliance Overview.

Big takeaway: Unlicensed != Unregulated.

Unlicensed means that the users of your systems don't need a license and that would be the case if your device complies to FCC Part 15 (ISM or non ISM).

Regulation means that any device on the US market must go through the compliance testing in order to ensure it meets FCC Part 15 specification.

FCC requires to test all transmitters but not receivers. However, for receivers you still need to write a document (DOD) describing how your system is Part 15 compliant.

Now (fully passive) RFID is a special case because it uses passive backscatter. RFID itself is never mentioned in Part 15 but FCC has published for few memorandum on how they treat these systems. Essentially they do not treat the tags as active radiators which hence do not have to be tested. However, readers (being active readers and transmitters) have to be tested.

So, if you sell your product in any way in the US ("on the market"), it must be tested and certified. For internal setups and usage this may not be necessary.