Sure, but if the ballot booth provides real privacy... it's not hard to do. It's also a common method of proving you voted a certain way in order to sell your vote.

Or hey, you can just peer over their shoulder:

https://twitter.com/Gabbienain/status/796031055715246080

This morning when I voted I was surprised that it was hardly a "booth". It had sides to the screen to attempt to block it from view, but it you turn your head you could see the screen next to you - and their screen.