Rights. We may not like the way some choose to use them, but they have them.
Not many burn our flag because they are just assholes. Those people are deplorable, and live only to destroy things. These people need mental help.
Next in line is the artist and protester. The artist had best had a reason for such "art". Some instances, such as a movie, it definitely represents a dramatic moment. You'd never see Spongebob Squarepants burn the flag of any nation just for laughs.
The protester, OTOH, has come to a point where he has been pushed to the limit. The flag burning is not the beginning of the protest, but the the " grand finale" to say that the government hasn't heard them.
The final flag burners ate those that assist in the American Legion. They believe it not proper to fly a tattered flag, and have a special ceremony to burn those flags that have fulfilled their use. Unlike those above, their ceremony is very reverent and solemn.
Four different groups, four different reasons. Legionaire, protester, artist, and asshole, all share one thing in common: they all have the right to burn the flag as they are fit.
And we do have the right to dissent their actions.