There's already federal laws for the proper use of the flag; you cannot raise the flag on any flagpole after it has been worn or touched the ground. And when the flag is raised, it must be raised up faster than the state flag, and should only be raised when all attention is to the flags. And the pledge of allegiance must be said. And only authorized organizations (such as the Boy Scouts or the military) can ceremonially burn the flag whenever the fire is starting to die out.
The American flag is a sacred symbol of America, and many, many people have died protecting that flag and the values that it represents to the American people. When people even think about burning such a sacred symbol, it takes away from the Americans who have died in wars. It's just disrespectful.
In this country, (America) we have freedom of speech and that means freedom from persecution from the government. Some people simply don't like things that the country stands for and want to protest with a flag burning. Those people should not be silenced by their government, or the Government would just be enforcing an act that goes against an Amendment of it's own constitution.