You cannot prevent that and it cannot be policed.
Now twitter has multiple apps, multiple platforms including desktop. You'd need some absurd DRM that would be broken anyway to enforce this.
Does this mean that the Snapchat screenshot detection doesn't work if you don't give it the storage permission?
But certainly it will not be a fluid process currently. Let's see if they update T&C's to accommodate that aspect.
That happens all the time, so yeah - it is a tool used to bully and harass, used all the time upon twitter.