Rogue elements in the scrap-metal trade are the real problem. Without the no-questions-asked deals such thefts would see a low return for the efforts.
About £5 a Kg for mixed Zinc compared to £9.70-£10 per Kg for Aluminium drink can scrap. [prices derived from £ per tonne]
Current scrap metal pricing...
http://www.letsrecycle.com/prices/metals
Again, if Councils would publicise their earnings from Al. can recycling it could go a long way to bring the non-recyclers in from the cold.