I donate blood every 8 weeks, and I go see movies about once every couple months. I earn 'points' that I can redeem for free movie tickets by giving blood. This makes going to the movies cheaper. If I didn't earn points, I would still donate blood, and I would still go to the movies.

What BitVPS is doing is similar. They are offering a discount for people who want a VPS, and also want to donate to slush. Without this, they would probably still be getting a VPS, it's hardly an impulse purchase market afaik.