Well, a price is set by the seller based on what they feel is their maximum amount of profit after considering supply vs demand, expenses, etc.
That price would not go down as sales volume increases. Any businessperson who would drop the price at the same time as sales are dramatically increasing won't remain in that business for very long.
However, if many balked at what the public generally viewed was too high of a price for an item, then sales volume would suffer and pricing would then drop.
All that is to say that I agreed to pay $25 for my copy of the game, and I liked the fact that I could send a free copy to a buddy so he could try it out too. But I viewed the extra copy as a bonus, and to be honest, it played no factor in my purchase decision, since being one person, I can only play one copy of the game.
That being said, "bundling" happens all the time in just about every sales environment. Think about the first Nintendo systems with the Mario game, or the Wii with a "free" Wii Sports; or any of your infomercials that say "If you act now, we'll send you a second one absolutely free!"
I would imagine the game devs chose this path because the game depends so strongly on the multiplayer component. You need large numbers in the lobby to generate further interest. If your first batch of buyers started to play multiplayer to find hardly anyone online, the game would stagnant and future game owners would be less likely to stick with it, and ultimately word of mouth would dry up.
But, if you potentially double the size of online players by giving away a copy for every copy sold, you are more likely to build a spike of players online, so when new batches of game owners show up there is a semi-full lobby waiting for them, adding to the game's momentum.
Anyway, those are my two cents, although they are likely worth less than that
Edit: fixed typo