Do you know why? Because contaminated loads are rejected by the tips, or they fine the council a hefty amount for it.
Councils don't just invent these rules to p*** you off, ultimately it all comes from government and their obsession with the 'green agenda'. The people that deal with your waste after the council have picked it up will use their rules to impose fines and sanctions on the council.
Yes but there is an element of common sense involved. You can throw your leftover sprouts away from your Sunday lunch, but you're damned if you put any leaves in your bin!

