And what's preventing you from aiming your phone at a monitor? Sure, people might notice if you're aiming at your $200 monitor, but with a high DPI, high brightness, wide color gamut monitor you can probably get away with it.
Analog filters exist, too. They might not be as great as their digital equivalent in some cases, but people really want to fake #nofilter and it's basically impossible to detect.
What? That sounds really stupid. Could you provide a source?
Even if Instagram does that, they can still modify the app to take pictures properly. That's definitely not a limitation with Android.
The only problem I can see with this is people modifying the picture header to become the edited photo instead of a legitimate one (either with a rooted phone or a modified request on pc). There's not really any way for Instagram to properly verify this.
E.g. people who prefer another camera app, or photographers who use a camera instead of their phone.