The Fed Gov does condone . . .
...provided you paid for the American flag you're burning at a duly licensed demonstration. Symbolism, using private property, is more than acceptable, it's legal.
The event in question, however, involves city property, and is therefore a city crime, one which should be able to call in all levels of investigation, from on-site cameras to DNA fingerprinting.
And as for the vulnerability of that bridge, the security is likely not as it was, just a few days ago.