It's a combination of two things, the subversion of expectation, which is a common device in jokes, and the criticism of authority, which is another, similarly common, one.

In my comment, I'm pointing out that these cameras are a risk for even broader abuses than just subpoenas, and compliance with law enforcement requests under the guise of finding lost family members.

The responder humorously twists my words to claim that it's not a problem because the lost family members have already been swept up in an indiscriminate drag net, such that the cameras can not find them.

The joke is on the DHS, and not it's victims.