In the UK there's always been some kind of import duty, plus VAT (Value Added Tax - currently 20%) on imports from outside the EU. However, there's a threshold level below which it doesn't apply, and a small quantity of electronic parts usually falls below the threshold.
For example, the JLC PCB's I've been ordering, the 10 100x100 boards for $2 offer, even including the DHL shipping, falls below the threshold. When I ordered 100 boards, this was above the threshold, so we were charged VAT, plus a charge for collecting the money, and 'perhaps' some kind of import duty as well. I'm not even sure what the threshold is
I presume something similar will apply in the USA, where small amounts aren't worth collecting?.