When returning from a vacation in the US VI, why did I have to go through U.S. Customs?

Question

Isn't the USVI part of the United States, albeit an unincorporated U.S. Territory? So why would I have to be cleared through U.S. Customs?

Answer 1

Same reason you go through your own door when you go back to your house.

Answer 2

Anyone entering the mainland through a port of entry has to go through customs, whether or not they came from a US territory.