Because according to Otasuke, it is, and its partner in romantic crime? Glico, the company behind such snacks as Pocky.
The promotion is actually a bundle package that includes the game, pocky, and a Valentine's Day card from one of the three female characters in the game. The bundle costs ¥6115 (about $68 USD); the original game by itself-- currently "sold out" --is ¥5800 (about $65), so if someone was looking to pick up the game anyway, it wouldn't be an expensive upgrade.
But for those who already own the game it would be WAY cheaper to go buy some pocky at the store and print out your own V-day card. Eesh.
Though most likely the team is oblivious to our presence, and probably just charging forward with love plus plus plus.
@gia: And also the fact that general Western audiences are not familiar with... Well, dating simulations like these. I mean, how did Wii get so popular with everyone? It's because it appeals to a larger range of people, that's why. I'm not very sure how Love Plus is going to fare in the Western market...