A. You are the exception. I am willing to bet .1% of users would be a overestimation of Twitterers willing to pay.
B. Setting up premium accounts and a payment system would probably run fairly close to break even. Not mention the additional customer service that would be expected and the complications that arise from splitting the userbase.
C. You aren't the real money. Twitter wants Coca Cola's money, CBS's money, etc. They don't care about pocket change from Joe Blogger. They are selling a package of 200 million people straight up, not a package of 200 million minus the most engaged and loyal users.