In short = no.
If there's nothing wrong with taking pix = no reason to delete them.
If there's something wrong with taking pix = no reason to delete them = they are evidence of your violation of their rules .

Can they prohibit you from taking pix onboard? Yes - they're a private company and you're on private property.
Could you get in trouble for taking pix? Probably -- but only with UA. No laws about it and law enforcement can't do anything about it. Can UA sue you? Sure.

You could get into trouble for failing to follow crewmember instructions, which is a federal offense.

In no case should pix be deleted .