I had the same situation go on in Gamestop. I had preordered it and was really looking forward to playing it, then once I purchased it I read the back and it said One Player and while still in the same building I told them (they were nice about it) "I didn't notice it didn't have co-op. I don't want it, I just want a refund." The ONLY reason I even care about using xbox 360 or PS3 over the PC version which has been out for so freaking long now, is the simple fact that you usually CAN play co-op. Honestly, it's so stupid to do this, and EA Games looks like it's going to hit rockbottom soon anyways (since Spore flopped because of 'draconian DRM' they're stock prices have fallen since about $46 a share to $16 a share and still continue to drop and now they re-create a title that has been around for 16 months now. If I wanted to play Sims 3 with no co-op I would get it for PC. I mean really, they already have add-on packs for the PC version, they already have new items, new places, new everything. So now they want to make a game 16 months later as a rip on a game that's over a year old? Either co-op or no sell like OP said.
My final argument, playing Sims 3 with your spouse (wife/husband, boyfriend/girlfriend) or a friend is fun and social because you discover things about each other, playing Sims 3 by yourself just seems depressing and lonely.