There is a big swap meet located in San Jose, California. There are at least 20 vendors in there selling counterfeit jerseys and other paraphenalia. Someone is getting that stuff into the US.
There are a lot of counterfeit goods seized at ports and borders so it can be an expensive gamble for any importer to try to get fake goods into the U.S..
Although some counterfeits will pass through customs unnoticed, many counterfeits are found but intentionally allowed to pass through so that federal agents can track the goods. The importers, the dealers and the sellers will then all eventually be arrested and the remaining goods confiscated and, often, destroyed.
In other words, there are retail stores that might be selling counterfeit brand name items, and it is not unusual to find counterfeit goods sold at flea markets and online on sites like eBay. These counterfeited goods are often tracked, starting with the importer to the dealer and then to the vendor and the customer. The large raids and seizures of counterfeit goods that occur in the states are the result of the careful monitoring of the counterfeit goods from port or border entry to the marketplace.