The law is not a toy, and it's not supposed to be wielded casually. The DCMA is certainly not treated with the respect it is supposed to afford citizens, and this is just another example of that.
It was a mistake for them to use a DMCA request here, because the code was MIT-licensed and thus even the author can no longer ask for it to be taken down. But nobody paid legal fees here, nobody was sued, and the code is back online, so, no, I am not amenable to the idea that Dropbox is being abusive.
Continuing to file DMCA requests would be abusive.
However, requests should be filed in good faith, under the understanding that you have standing to file the requests. Clearly Dropbox does not, nor does the original author, having MIT licensed the code himself.
That pretty succinctly summarizes why this is problematic. Everyone here knows that they can't do this. The fact that they prepared letters and fired off the requests anyway demonstrates that they were acting in bad faith.
You don't accidentally reach for the DCMA and accidentally shoot off requests to github.
(and it's on that count alone that i criticize Dropbox, i think otherwise that i totally understand why they think this is a huge problem, and exposes them to legal liability.)
Edit: just read (http://news.ycombinator.com/item?id=2482803 ) explaining that the DCMA takedown letter that was email was in fact accidentally sent via an automated system. :P
Abuse of the DMCA is already dramatically overblown, and the cavalier attitude of "oh well, deal with it" is a significant part of the problem.