It's entirely reasonable to create a system designed to allow people to share things once, and indicate to others that they don't want it spread any further. It's reasonable to design software which attempts to honor such requests by introducing friction, making it 95% effective despite knowing that 5% of people will be able/willing to work around it.
Most people are "path of least resistance" and are too lazy to work around something like this, which is exactly the point. Pretty much every person using such a system is aware that the recipient could just take a picture of the phone with a camera.
Perfect is the enemy of the good.