This is coming out as an expansion after the initial release of the game. Or at least that's what the developers are promising if the initial launch goes well.
Indeed planetary landings (and walking within a space station) requires as much work, if not more than, the space-flight stuff, so it's understandable that they don't want to just tack it on.
Frontier and First Encounters games all had seamless planetary landings so the goal has always been to include them in Elite: Dangerous as well. Everything the developers have promised so far has been realized (although some things a little later than initially scheduled), so thus far it's a good bet that they are not overselling their capabilities.
