It's probably not so simple on Riot's end. Lets think of what needs to happen for "simple and free" transfers on Riot's end:
- Usernames have to be checked on the destination server before moving. If there's a conflict, the "simple" solution is to assign a default username to the player, such as "player_25367". Needless to say, not ideal.
- A system needs to be put in place to preserve usernames after the transfer, so that when you transfer back you're not stuck with "player_25367" because someone liked your username and waited for you to jump servers.
- Player's Runes/Masteries need to be moved from one server to the other. For an individual player, it's probably not too bad, but compound the problem with the number of people you have transferring back and forth each day this adds up.
- Player's Content needs to be moved from one server to another, including the purchase history and refunds available. Riot needs to move this because first and foremost they are a business, and if they can't keep track of your transactions they'll lose your business.
- Match History needs to be transferred back and forth.
- Player History needs to be merged between "Trusublime" and "player_25367", since it's the same player in both cases.
- Match History needs to be adapted so that it can access players on different servers in the same history.
And this is just the high level list of what has to happen. That's not taking into account the deeper "how do we actually do this?" discussion. And no, the current Migration system isn't good enough, since it's basically meant for one-way trips, not repeated round trips.
Then you have to set up a separate data center for the servers, complete with it's network infrastructure and IT personnel. Add to this the extra time Player Support has to put in to explain to people why their old username is taken and they're stuck with "player_25367", and it is definitely not "free" to Riot.
At the end of the day, while it's a bold undertaking, the roadmap that Riot has set out for the NA Server is a whole lot better than simply standing up an East Coast Server.