Guys who poach become known to conservation officers. If I was hunting deer, and selling it, sooner or later I would come to be known, and then you could bet I'd be challenged to the letter of the law. But I don't sell a deer a week, so anything I have in my freezer wouldn't be picked over.
I give away meat all the time, I don't bother to tag it. But, I should. You are right in that it is technicaly the law that it be tagged if it leaves my possesion.
The law however, isn't really intended for that, it's intended to catch pouchers.
BTW,you'd be amazed at the authority and jurisdiction a federal conservation officer has.It's much like the FBI and far broader than local or state police.They even have the right to issue speeding tickets.Search and seizure of your home,vehicle,possessions,etc.,,are also under their authority.Not that it happens much.Usually probable cause,as in the case with the guy caught in the act of poaching,is warranted before they search your home.
I was with my uncle when they(a group of 3 officers) caught a deer poacher.His gun and vehicle were confiscated.He was sent to jail and fined.His vehicle was later sold(a brand new full size Ford) by the state,but he was still required to pay the payments.Game laws,for us common folk,are serious offenses.