de0u I doubt that Apple or Google decided all on their own to add this ability. So if app authors want such a restriction and can disable their apps on platforms that don't enforce those restrictions, and if users want to use those apps, what comes next?
The thing is, app developers don't want to lose sales. They are using this feature because it is available, but they would never remove their app from Android or iOS because they don't implement a way for apps to disable screenshots. WhatsApp is available in a web version, where they obviously cannot enforce this. Netflix is available on computers without any secure DRM, where capturing every frame would be trivial. They use these features when available, but they don't remove their app if it is not available, because that would hurt sales too much. So Apple and Google implemented screenshot disabling and secure DRM to please these app developers, not to keep them on their platform.
de0u I'm not saying I know what "the right answer" is. But it's also not clear that deploying one reasonable principle (such as "It's my device so I should be able to do whatever I want with it") without balancing it against other issues is always the most productive approach.
Maybe so, but I also believe there is a lot of value for us end-users to hold on to principles and rights we should have against app developers and providers of services, or they will run over us. Like how consumers have literally no rights towards copyright holders in today's society. Very unbalanced situations like those are obviously not good, so I believe there is value to hold on to principles that guarantee our end-users freedom too, not just app developers and service providers freedom. We need balance.
My preferred messenger, Element, has a download button next to every user's avatar, where you can download the avatar in full resolution. I am already not recommending WhatsApp to anyone, because it isn't open source, but I also prefer to have my contacts on Element whenever possible. And I strongly prefer streaming services that allow DRM-free downloads as regular .mp4 files, and consciously prefer to use them, and discover and watch what is available there instead of on services that lock things down using DRM. Not that I ever download anything, but I want to support the streaming providers that allow that, because I am against DRM and overly restrictive copyright. This is also why I prefer to use purely open source operating systems, where I could remove any freedom restrictive technology added if I want to. And if a service provider then choose to not allow me access to their content because of that, fine, I wouldn't want to support them financially anyway in that case. So I want the OS I am using to put some pressure on app developers and service providers to respect my freedoms. To maintain a healthy balance in rights between content producers and content consumers.
(I am against GrapheneOS trying to get Google Play certification for example, because that would encourage app developers to use freedom restricting technologies, and no longer discourage GrapheneOS users from using those apps.)