First of all, I always thought AK-47s were illegal outside of purchasing them from collectors. I thought they had to be made before 1986 or something like that. Nothing positive comes from carrying something like that to a rio-errrr, protest. Unless the British are trying to quarter soldiers in your town, open carrying an AK-47 is retarded.
OK, let me break this down for you:
Fully automatic weapons have been heavily regulated and controlled since the 1934 National Firearms Act. In order not to violate the 2nd Amendment, the regulations are couched as "taxes". To legally purchase a full auto firearm, or many other weapons that are "Class III NFA" regulated (short barrel shotguns, grenade launchers and so on), you must pay a $200 tax, submit to background checks, ATF will open a file on you and monitor you for the rest of your life and fill out a mountain of paperwork and then wait six months.
In 1986 Ronnie Raygun signed the McClure/Volkmer Firearms Owner's Protection Act, which, in exchange for certain "protections" banned the production and sale of all new automatic weapons under NFA regulation. (That is the only true firearm
ban ever passed.)
None of that applies to what I am sure this guy was carrying...a SEMI auto rifle, which is not even a true "Avtomat Kalashnikova" as it is not "avtomat" or automatic.
Now, ATF waffles and flips-flops and throws up clouds of regulations that limits importation of certain classes of firearms as "having no sporting value", getting a fully assembled, semi auto AK-47 clone is difficult at times.
But if you assemble one from parts and use more than 10 US made parts in that assembly, it will pass muster for import.