Law formed in some states as a deterrent to littering the streets with empty bottles and cans. The idea is that every can and bottle sold in the state has a small surcharge placed on it which is refunded after they are taken back to a bottle return.

The charge is relatively small; in Michigan, it's 10 cents per can or bottle of any carbonated and/or alcoholic beverage. In other states with such a law, it's 5 cents. (I'm not sure about restrictions on beverages in other states; I believe Maine has deposits on non-carbonated beverages, too) While small, it proves to be a sufficient incentive to make people stop throwing their bottles on the ground.

It not only helps the states that have such a law, but neighboring states (especially those that are near Michigan, with the highest deposit) benifit, too, as people try to take bottles from their own states, without the deposit on them, into states with deposit laws, in an attempt to get some free cash. The problem with this is that the bottle deposits are paid out by the state (in the end), and said state does not get money from deposits from outside states. Thus, the bottle deposit laws restrict returns only to 'authorized' bottles and cans, that is, ones with appropriate deposit stamps on them. Any bottle return worth their salt is supposed to be able to catch, identify, and reject 'unauthorized' containers.

The only problem with bottle deposit laws is that they give rise to the bottle return...

NOTE: If any of this doesn't apply to your state's bottle deposit laws, let me know.