As with most any electronics, whether it's a drone or a computer or a radio, if it doesn't have FCC certification, it is not legal to use or operate in the US. There is nothing to actually stopping you from using it but you could be in serious...
A White Paper by Pilot Institute
Read the story: Pilot Institute Survey: 8,056 Operators Reveal DJI Ban Will Devastate American Drone Industry
Download the document: The Effect of Banning Affordable UAS in the United States
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These two FAQs together set the stage for bigger discussion (still leaves an open question):
Some of the UAS critical components (for instance, batteries) traditionally do not even require FCC equipment authorization. Do they now?
No device...