What "No Purchase Necessary" Actually Means
The phrase appears on every legitimate US sweepstakes because of how the law separates three things that look similar: a lottery, a sweepstakes and a contest.
The three-element rule
A lottery has prize, chance and consideration, meaning something of value given to enter. Only states may run lotteries. A private promotion must remove one element, and removing consideration is what makes it a legal sweepstakes.
The alternate method of entry
Where a purchase is one route in, the rules must offer a free equivalent with the same odds, often by post. That is the alternate method of entry, and a company offering it is following the law rather than being generous.
Contests are different
A contest awards on skill rather than chance, so it may legally charge an entry fee. If you are asked to pay, check whether you are being judged on something, because a paid game of pure chance is not lawful.