It's illegal because technically it's stealing.

There is an owner - the council. People give it to them when they put it out for collection, usually on council land (the nature strip)
There is an intent to permanently deprive that owner - you're going to keep it/sell it/recycle it
You don't have their consent - you can't, cos who in their right mind would ask the council? lol

Our tip has a tip shop too, but they don't do hard rubbish collection because it costs them too much money, so our rates would go up.

However, I'm all for collecting Great way to pick up some neat stuff.