I don't think anyone here has suggested making Steam compulsary. You can easily have Steam sold games and LFS website sales kept seperate, without impacting the game or separating the community.
Many games today are purchased on Steam and retail, (Bad Company 2, Call of Duty etc), both versions can be played together with out any issues, and for certain games retail customer could integrate their CD keys to Steam anyway to get the benefits. Selling this game on Steam would be no different to selling it on Amazon or your local Game store, it just so happens that Steam is a popular purchasing platform with huge number of customers.
And to Psysim's point, how would the game change? It would change in no way what so ever, the issue you point out is non existent. No one would force you to use Steam, thats not how Steam works. Also, it wouldn't be a sell out at all, a lot of indie game developers use Steam to distribute as it has a massive audience, and they maintain their indie status.