Because they didn't lie. They never advertised TDB, HoW, or TTK as part of the game at release. TDB and HoW probably weren't even finished when Destiny launched, and I doubt TTK had made it past "oh here's a Dreadnaught playspace." Does it mean that Destiny didn't have a major development shift mid-production a la Halo 2? No. Does it mean that I'm entirely happy with Destiny and Bungie's/Activision's business practices? No. But I don't let it hamper my experience playing Destiny.