He wasn't sentenced for burning a flag, he was sentenced for destroying stolen property. The first amendment doesn't enter into it. If he brought his own flag and burned that, there wouldn't have been a legal issue with it.
Edit: since I'm "posting too fast"
>has some value.
https://www.amazon.com/s?k=black+lives+matter+flag
It's a $20 flag, and we paid for his rent and meals for five months over it. He could have simply been court ordered to replace their flag, but instead we spent a mountain of tax money feeding and housing him. He won. And the 1A lost at the same time.
Serial offender who was convicted of theft in 2004, at the age of 20; convicted again of felony theft in 2012, at the age of 28 (for which he was sentenced to 30 months in prison); and convicted against in 2022, at the age of 40, for his violent role in attacking the US Capitol (for which he was sentenced to 22 years, and pardoned).