I don't think that Amazon is worried about people using this to strip the DRM and share the books. They just don't want total strangers using websites to lend each other books.
CNet claims they railed 2 other sites as well, but didn't specify which ones:
http://news.cnet.com/8301-1023_3-20045633-93.html
I'm not surprised this got shut down -- in fact I'm surprised it didn't happen earlier.