How to save a shared video to my Camera Roll on iOS? I have the app Photos on my iPad mini, on iOS 7.

I am viewing a stream shared by a friend. 

When I am on a photo, I can save it to my Camera Roll. 

But, when I am on a video, I cannot save it to my Camera Roll. The feature is missing. 

1) How can I save a video to my Camera Roll ? 

I want preferably a solution that do not need a computer.

2) Bonus question : Why has Apple limited this feature to photos ?
Apple doesn't have a solution on iOS to accomplish this. I've not found any third party app from the App Store that will read the shared stream videos or receive a paste event (you can copy the video from Photos app on iOS, but pasting it anywhere seems to not work for me on iOS 8.x)

I use iPhoto on OS X to view the shared video and then import them into my library. From there, I can get the videos onto iOS. Not exactly your preferred solution, but it works well enough for me.