Talk is cheap but in a way this is what Google did with AdWords. In the past you should have a budget of thousands dollars to advertise on popular sites like Yahoo. Google found a way to keep the required budget at a minimum.
I think Twitter haven't done that because it is always more comfortable to talk to a few customers who can pay a lot of money instead of having an horde using your service, but it is currently unacceptable to think this way if you are Twitter.