Well.. I'd say: Why not? The colony on Greenland was important in order for the Vikings to settle Vinland. In other words, if they hadn't settled Greenland they hadn't settled Vinland.
Because quite simply, when looked at from a game perspective, the Vikings have got enough on their hands already.
Not only do they need to be the first to colonise the Americas, but they have to also eventually deal with an aggressive Germany and Russia back home, while trying to maintain a large sum of money. Forcing them to settle poor cities in order to perfectly emulate history makes this a simulation, not a game.
Anyway, from a historical perspective:
Iceland is an integral part of Viking/Scandinavian/Danish history and maintains and important role today.
Greenland's Scandinavian colonies failed initially and were later isolated continually from the world until after the second world war. Greenland's entire population today (50,000 +) is barely as large as many of the ancient cities we play with in Egypt/Mesopotamia etc.
There is simply no justification to push this requirement onto RFC. There are countless (and I really do mean countless) other countries who could also lay claim to such tiny additions based upon historical situations.... they can't all go in the game. We can't build the map to ensure that every civ gets the exact same cities in the exact same places at exactly the right time, or we wouldn't have a game - just a simulation.
Finally, there is absolutely no proof of your statement. The only way that Vinland has any connection to Greenland is that potentially, people were trying to sail from Iceland to Greenland, got blown off course and ended up settling Vinland..... that hardly constitutes being an integral part of the settlement of the Americas!