The people using this feature aren’t going to imagine it as 100% protective against any and all possible methods of exfiltrating data. It’s a feature for discouraging casual data capture and dissemination. It serves to remind people doing spontaneous captures that they aren’t supposed to, but everyone knows a dedicated person could find a way to get that photo if they really, really want.
That doesn’t make it useless. Every time you raise the level of difficulty for accomplishing something, a percentage of attempts are thwarted or discouraged. As it turns out in the real world, raising the bar even a little tiny bit is effective in thwarting or discouraging the majority of attempts across the average user base. You’re not defeating the dedicated attackers, but you’re reducing the overall number of successful attacks and even attempts.
The same is true for things like the office firewall blocking websites: Yes, we all know a dedicated engineer can create a way around it, but it’s going to stop most employees from getting to those sites and serve as a reminder to others that they’re not supposed to access them.