What does postal voting have to do with that? they could also force you to film yourself at a polling station...

Well at least where I live (the UK), it's illegal to film in a polling station and the staff in the polling stations should be looking out for that and reporting it.

More importantly (with both postal voting and in-person voting) it's impossible to perform these attacks after the fact, but if you have a receipt of your vote it can be done at any time after you vote.