The DEA prides itself on big hauls and tends to leave small stuff to the locals. Success in their line of work is measured in kilos and tons.
I can see them making a bit of noise about SR crackdowns to discourage use of the site, but it's hard to imagine them dedicating a ton of technological resources to a relatively tiny number of people selling each other dime bags over the Internet.
I could be wrong, but it seems like it would be a waste of a lot of time, energy, and money for very little return. Granted, that statement could apply to all drug law enforcement.
I think you hit the nail on the head regarding "waste of time, etc...". The reason silk road isn't going anywhere anytime soon is the size and scope. It is naturally restricted to a small internet community (relatively speaking) due to technology requirements.
Getting there with the Tor network is pretty straight forward but then going through the trouble of learning how bitcoin works, how to get funds from your hands to an exchange, how to then get the bitcoin to your wallet, then to SR will make your average user's head spin. Then throw in the fact that most seller's require PGP for messages within SR and the user now has to learn to use PGP ...suddenly the average user's head explodes. Easier for them to find some shady friend of a friend to get stuff from on the street.
This limited flow on SR would be like the cops wanting to spend hundreds of thousands (if not millions) of dollars in man hours and technology to bust a a few kids selling playboy magazines to a bunch of 15 year olds when real life politics and public relations force them to do the morally superior thing, i.e., spending those same resources hunting down and prosecuting peddlers of child porn.