I tend to agree - if panes are freezing over I don't bother working, or charge for what i've done if i've started and it all starts to go tuts up. I've had it a few times and expect more this winter. I also worry about damage to pumps and filters in the van if its massively cold.
Of course it does depend a bit on which window was frozen over and also if you are 100% sure when it melts it will be clean. If it's a garage window or one of 30 on a house its a different matter (and there again it wouldn't be a tenner) - i'd expect payment, perhaps less a quid.
Notwithstanding this these people don't appreciate your services simply because of the way they spoke to you and treat you on a regular basis (i.e. forgeting gates - once is OK, but a serial offender... nah); treat people as you expect to be treated I always say. If they had bothered to call and speak to you you might have had a chance to explain.
I'd bin 'em and replace.