I feel it is within their rights to burn it but it does raise the question of why they are still here when they are so angry with the country as a whole. To me, it is a rather hypocritical statement to make when you are living in the country whose flag you are burning. If the country is that intolerable to you then move somewhere else as no one is asking you to stay and no one asked you to come either for those who moved into the country.
As far as outside the US goes, It becomes less hypocritical because the people burning it are receiving direct benefits from the country whose flag they are burning but it's not totally without hypocrisy as many times the nation in question does receive aid that they benefit form if only indirectly. I'd say if they hate the country enough to burn its flag then they should desire to be cut off from all aid and benefits offered by that country whose flag they are burning. To do otherwise demonstrates that your statement made by the burning of the flag is a hypocritical one that has been stripped of any meaning by your willful hypocrisy.
Maybe they're still there because the government or economy is awful and they barely earn a living wage, let alone enough money to afford to move to another country? You seem to be disillusioned with regards to how much work it takes to just up sticks and get out of dodge.
Do some people really take more offence to the flag being set alight than they do to say, veterans living on the streets because of poor care by their government, or the ill treatment of coloured or transgender people living in a country whose ethos is proclaimed to be 'and justice for all'?
Personally I have little issue with flag burning. If you bought the flag, it's yours. Set it on fire, wipe your backside with it, erotically asphyxiate yourself with it if that's what you're into. It's your property, do as you like. Symbolism be damned.