It's a health and safety thing: if stuff isn't in a wheelybin they'd have to lift it into the van, which isn't designed for it. They use a different sort of van on the rounds where they pick up bin bags (there are some places where people haven't room for a wheelybin). I've always wondered why they can't empty the bin, lift the rest into the now-empty bin and then empty it again. But you might be right, I just don't know of a rule which says only one bin per household.

I'd go poke around the council's webpages and check but I seem to be a complete lack of arsedness today.