I can imagine, if @XOIIO is travelling by car, the border scenario would go like this:
Border guard: How long have you been in the U.S.
XOIIO: a week
Border guard: Did make any purchases?
XOIIO: just these pop-its.
Border guard: What are they?
XOIIO: The explode when you throw them on the ground.
Border guard: Can you please pull your vehicle over to the side.
It seems that you would be ok if you read The first link in this google search which is a PDF document They do seem to exempt small amounts of caseless gunpowder.
To be absolutely sure you should call the Border Services Agency.