Steven Nelson (U.S. News & World Report) has the story. Here is the police department press release:

On the morning of 7/4/16, the Urbana police department began receiving calls concerning a Facebook post that portrayed Bryton Mellott, a citizen of Urbana, burning an American flag. The images and narrative in the post caused some to call and request police action against Mellott and others to call and express concern for the safety of Mellott and those around him. Officers viewed the post and saw that there were a rapidly growing number of social media responses. Many threatened violence against Mellott and his place of employment, which fielded a large volume of calls regarding the post.
Given the volume of responses and specificity of threat against his place of employment (a location where an act of violence would likely cause harm to others), prompted police involvement in this case. After investigating the incident and speaking to both Mellott and his employer [Walmart -EV], Mellott was placed under arrest for flag desecration. The police report lists Mellott as an offender of both flag desecration and disorderly conduct as well as a victim of disorderly conduct. After consulting with a member of the States Attorney’s Office, Mellott was released from custody and given a notice to appear in court. Mellott’s release was due to questions about the constitutionality of the 2013 Illinois flag desecration law.
The Urbana Police department recognizes that this is a case where the right of free speech comes into conflict with the safety of uninvolved citizens. The actions taken in this case have been to try to assure the safety of the public and Mr. Mellott. The Urbana Police urge the public to express themselves in a peaceful way and to not retaliate against unpopular speech.
Lt. Joel R. Sanders
Patrol Lieutenant
Urbana Police Department

But the Supreme Court made clear, in 1989 and 1990, that flag desecration laws such as the Illinois law violate the First Amendment. And the court has made clear since 1949 that the government can’t punish someone for “disorderly conduct” simply because his speech offends people and leads some of them to threaten violent retaliation. The police must protect the speaker (even though such protection understandably involves cost and risk for the police), rather than criminally punish him for his speech, except perhaps in some extremely rare cases that involve brewing riots on the street — a narrow category into which this speech doesn’t fall. And indeed this has to be the rule, or else any online speech that offends people enough (whether anti-American speech, anti-Islam speech, anti-gay speech, pro- or anti-Trump speech, and more) could lead to arresting the speaker.

To her credit, Champaign County State’s Attorney Julia Rietz announced today that Mellott won’t be charged:

The State’s Attorney’s Office is declining to file charges against (Bryton) Mellott as the act of burning a flag is protected free speech according to the US Supreme Court decision, Texas v. Johnson, 491 US 397 (1989).
We have considered 720 ILCS 5/49-1, Flag Desecration, an Illinois statute currently in effect. This statute was the basis for the decision by Urbana Police officers to arrest Mellott. While that statute remains in effect, it is contradictory to the US Supreme Court ruling in Texas v. Johnson. We will be discussing this issue with our local legislators and asking that they consider reviewing this statute given the constitutional issues it presents.