You can buy meth and heroin off of SR. Who cares if some 11 year old gets drunk. He can go through his parents' liquor cabinet for that anyways.
And at this current time, drugs and gambling make up the bulk of the bitcoin economy, so blocking vices does more harm than good...
Whilst I agree with the first part, but I think you underestimate the number of people using bitcoin in legally accepted ways.
From when it started until the beginning of this month, Satoshi-Dice had been responsible for 54% of all Bitcoin transactions.
So I think it is a pretty safe statement that drugs and gambling make up the bulk of bitcoin
transactions.
For the more general statement, it depends how you define economy.