just watched First Blood for the first time an can someone explain why the cop hated Rambo?

Rambo is just walking down the street and the cop at one point says "wearing that flag on that jacket, looking the way you do, asking for trouble around here"

the flag was an american flag, he was wearing it in an american town.

the jacket was an US army jacket, i think.

why would he assume Rambo's a drifter? what was his problem?