I've never read Haidt but I am very familiar with the concept of flags as religious symbols and flag ceremonies as acts of worship to those symbols.
West Virginia State Board of Education v. Barnette, 319 U.S. 624
Justice Jackson's majority opinion in that case acknowledged the flag as a symbol and saluting the flag as symbolic speech. Hence refusal to salute the flag is protected under the First Amendment.
http://www.uscivilliberties.org/historical-overview/3803-flag-salute-cases.html
*MySmiley*
"Bustin' makes me feel good!"
Ghostbusters, by Ray Parker Jr.