Taking a photo of your ballot paper is a criminal offence (at least where I am) and one which can only be committed at a specific physical location and time. In other words, it's a crime with a very low payoff (an individual vote can't be worth very much money), and a relatively high risk of detection.
The risk of getting caught taking a screenie in the security of your own home with no pollwatchers and election officials around is much, much, lower.
The risk of getting caught taking a screenie in the security of your own home with no pollwatchers and election officials around is much, much, lower.