You have answered your own question!
Not that I am in the US, but the situation is the same.
You are buying "cheap stuff",. Below a certain amount, it would cost more for them to attempt to collect duty on the items, than the duty collected. The amount is generally about $1000, but may be less. Certainly, collecting 10% duty on a $100 item is barely worth the paperwork (including opening the items and estimating value), especially as people are likely to dispute the value and it would just become bad "PR".