I was curious about why they were saying that it is mandatory, but not enforcing it so I looked in the DHS page. It appears that they are rolling the system out across the country now and that eventually, they will check your receipt when you board your flight. So it's not REALLY mandatory yet. However:
I wonder what "affect their ability to re-enter the country" means. When I leave, they take the visa stub from my passport that confirms that I have left the country. Although US-VISIT maybe more efficient, it seems redundant with the data they are collecting from the visa forms. I suppose the US Government doesn't have to have much legal or logical requirement to explain what "affect their ability to re-enter the country" means.
Checking out of the country using the US-VISIT exit procedure is mandatory where an exit solution is in place at the port of departure. If visitors fail to check out through these facilities, it could affect their ability to re-enter the country.
When I asked a DHS officer what I should do with the receipt, he said, "Keep it. You may need it later." I'm not sure what that meant, but I didn't probe further.
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