I think at one point they had plans to do this and anybody would have been able to create and sell sets. This would have been amazing. I think they have changed their mind since, years have past and they now use a kickstarterish system for electing user-created models that are made available to the mass market. Not quite as exciting, but still forward thinking.
Of course you can use their designer tool to create models and order them yourself, but it works with a limited range of parts.
Not the other way around.
Maybe this is reflecting the state of our society - we just like to see a cool brand, buy pre-made things and follow the instructions.
But I tend to believe there's a master builder in all of us that Lego can unleash, if they have the guts (not so subtle Lego movie reference, check).