http://www.straightdope.com/mailbag/mpurchasetoenter.html.
Great report Rico.
Thanks!
About that contest your radio station offered…
Wouldn’t you have been safe anyway, in that case? Presumably, the first person, or the first n people, to return all of the items were awarded prizes, correct? Wouldn’t that make it entirely a game of skill, rather than luck, so long as you didn’t randomly draw from the valid entries?
Nice report, Rico!
Ditto on the nice report.
As to the comment in the question that no one has taken advantage, there is an entire community of contesters, which my wife got involved in when she had a back problem. There was (and no doubt still is) a newsletter giving many contests and their requirements. She won a whole bunch of stuff, mostly minor things like sports bags, through sending in postcards.
As far as we could tell, the companies that run these contests do not distinguish between postcard entries and other ones.
There is a technical detail about this that I’m nearly sure is true. 
No purchase required means that no purchase is required to enter. It doesn’t necessarily mean that you get an equal chance to win without purchasing, just that you have a chance.
Suppose 99 people purchase a product to enter a contest, and 100 people take advantage of the no purchase required clause. It is legal to draw 1 name out of the 100 non-purchasers, throw it in with the other 99 who did purchase, and then draw again for the final winner.
Technically, someone who didn’t purchase could win this contest, but in practice, a purchaser has a 1% chance of winning, and a non-purchaser has a 0.01% chance of winning -0 quite a difference.
I seem to remember Publishers’ Clearing House getting majorly spanked for that sort of game-playing, just a few years ago.