See this article in Slate. I’ve written a fair bit about this topic, most extensively in my article, Vote Buying, 88 California Law Review 1323 (2000), and the Slate piece has it about right. The only thing I would add is that it is not clear that giving away Ramen and underwear in exchange for a promise to vote later really is paying for turnout rather than just an unenforceable promise and publicity stunt.
The more typical payment for turnout arrangements (as in California, where it is legal when there are no federal candidates on the ballot) require that the voter show a voting stub and then gets something—a free car wash, a chicken dinner, etc. Democrats have targeted these in Democratic areas as a way to get out the vote. (All of this is detailed in my article.) The Moore situation looks different, and I think would hardly be grounds for a prosecution.
I recall that during the 2000 election, my local car wash (which has a civic-minded owner) offered free car washes to those who showed a voting stub. I called the car wash to give the owner a heads-up. The manager thought I was threatening to turn him in for breaking the law and hung up on me, proving that no good deed goes unpunished.