I also flew in from Japan through SFO on Monday. I had to show my items to a Japanese customs agent, who wanted to verify that we were indeed taking them out of the country. Like
@lalame ‘s experience, the CBP officer in SFO also looked through the computer and could see my purchases both from duty free and from boutique.
It is very convenient that in Japan the VAT is deducted at the boutique/point of sale. There is no need to scan any forms at the Pablo or Diva machines as in Europe, and/or wait in line for a custom officer’s approval/stamp.