We inspect 5% of containers, so one container with 300K is the spillage of running dozens of containers into the US. Most counterfeits are used overseas, it is so bad that most foreign merchants will not take a US 100 dollar bill, and these are the ones in proximity to Us FOBs and bases.
That is a very misleading and erroneous method of approach to this subject.
We do track all ships coming and going from the USA. Only specific ships that are from specific areas or have deviated from predetermined schedules or have been flagged for one reason or another are inspected.
Much of the cargo that comes in is coming from known sources and thus are given more latitude.