I didn't see anything about requiring proof of purchase on Revell's site. Can you point me to the line in the rules regarding that, Dan?
From the Contest Rules:
Entrants must log on to www.revell.com to enter online. Buy one (1) new Revell or Monogram brand model within the Contest Period (February 1, 2013 – May 15, 2013). The list of eligible model kits can be found at www.revell.com or at your local Revell retailer. Complete the online entry form and submit it electronically along with up to five (5) photos of your model (up to 2 MB per image)."
The rules require that an entrant submit an online entry, which may only be submitted if the following is "checked:"
By clicking the checkbox, I verify that I personally built the Revell or Monogram plastic model submitted for consideration using one of the eligible Hot Rod models. I understand that I am also accepting all terms and conditions of the Contest. For a complete list of terms and conditions, see the Contest Rules.
Requiring proof of purchase? Maybe not directly required upon initial entry. I don't know if they follow up on the purchase requirement.
Sure, anyone could cheat. But, I'd be willing to bet that anyone selected to win any prize will be asked for that proof of purchase.
If not, that person might win a prize by cheating. Congratulations to them. They might as well have a "professional" builder construct their entry, too.
As has been mentioned by others, the major impetus for Revell to go to all this trouble is to promote sales. They even refer to it as a "promotion." No secrets there. It's their promotion/contest. And they are entitled to set their own rules for their own promotion/contest.
If entrants bypass the rule requiring purchase (i.e., cheat) and if entrants who did not purchase their kits during the contest period (i.e., cheat) are selected to win prizes, then Revell is also entitled to decide the promotion was not worth the trouble, especially if sales aren't sufficiently increased.
Even if proof of purchase is not required to be delivered at time of entry, it is a requirement of the rules.
Besides jeopardizing the whole thing*, following the rules is all about ethics, integrity, and character of the entrants. Those are attributes determined by conduct and behavior whether there is a 'hall monitor' checking receipts at the door.
*Disclaimer for this aspect of my statement: Realistically, I know ~ or at least I hope ~ that cheating by bypassing the purchase requirement rule is not likely to be widespread enough to persuade Revell to discontinue the whole thing. But it is possible.