It's the law.
From:
Our Flag, Too
By William Rivers Pitt
http://truthout.org/docs_03/040403A.shtml(snip)
The basic message behind an upside down flag is, "Distress. Send Help." The symbolism of the reversed flag began in the nautical realm; sailors at sea in distress would fly the flag upside-down before abandoning ship, or as a signal to other passing ships that they were in dire need of help. More recently, this symbol was used by troops in the field in Vietnam, who flew the flag in reverse to signal their distress over the manner in which they were being used by their commanders and political leaders.
In 2001, the use of the American flag as a symbol of distress was codified in law. U.S. Code, Title 4, Chapter 1, Section 8(a) reads as follows:
The flag should never be displayed with the union down, except as a signal of dire distress in instances of extreme danger to life or property.
(snip)
Combine the clear language of Title 4 with the unambiguous wording of the First Amendment of the Constitution:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or of the right of the people peaceably to assemble, and to petition the government for a redress of grievances.
(snip)
- Title 4, Chapter 1, Section 8(d)
The flag should never be used for advertising purposes in any manner whatsoever. It should not be embroidered on such articles as cushions or handkerchiefs and the like, printed or otherwise impressed on paper napkins or boxes or anything that is designed for temporary use and discard.
- Title 4, Chapter 1, Section 8(i)