Never happened to me and don't think it's normal if it wasn't a recycling bin as you said but a regular one. Their job is to collect the rubbish after all, not to pick or leave it there. If I were you, I'd politely ask about their politics and see if they can give an adequate answer. I am glad our bin men from http://www.rubbishwaste.co.uk/ are outgoing and would never do that without a reason.