Well, if away fans are throwing, inter alia, vapes and coins, at home fans sitting below them, it seems quite fair (to me) to deprive them of such objects prior to entering the ground.
Re. the confiscated coins, Sunderland have stated that this will be given to a charity of the away Club's choice.
The legal definition of Theft, far more important in my view, requires 'the coins' to have been taken ....dishonestly....
I am quite sure a sign warning away supporters that, should they endeavour to bring coins with them in to the ground, such coins will be confiscated (and given to a charity of their Club's choice) should absolve Sunderland of any allegations of theft.