If you have to buy a product to enter a draw, you're effectively buying a lottery ticket. It's illegal to sell lottery tickets without a licence, and licences aren't available for commercial lotteries. So the only way the companies who run these competitions can get round the law is to make it clear that you can enter the draw without buying the product (i.e. this proves that they're giving entries away, rather than selling them).
When companies first started to find ways around the law, they normally required that you had to send an entry, on plain paper, by post. These days, it's more common to permit free entries via the web. For example, you can enter the McDonald's Monopoly promotion, without making a purchase, here:
http://www.mcdonalds.co.uk/monopoly/
Chris