You can burn the U.S. flag and nobody can throw you in jail. The "speech" is protected, even it means you're an asshole.
You just called the US Marines and the Army assholes. Nice going.
From the Marine Corps Flag Manual, MCO P10520.3B,: "Flags determined not to have historical value will be destroyed by the parent organization, privately by burning,"
Army Regulation is AR840-10. Section 2-12, paragraph c, states that if the flag has no historic value, "it should be destroyed privately, preferably by burning,"
You do realize that burning is the proper way to dispose of a US flag, right? The flag must be folded in the traditional triangle shape and placed on the fire. If burning the flag is somehow not possible then it is permitted to bury the flag, again in the triangle fold. You can also shred the flag before burial. This actually is not limited to US flags, other nations have been known to shred and bury flags, particularly in tomes of war to avoid capture of the intact flag.