The question in the KB is almost spot-on, and the answer is verbatim, aside from them leaving out the code sample from the question. Is this something Salesforce has recently started doing? Should I just be flattered? SFSE got no love in the KB article - if it did it might drive more traffic our way!
Salesforce sources its Knowledge Base articles from things that employees find interesting and submit internally, after which it goes through a review process and eventually ends up being published. Employees are encouraged to submit new articles that they find interesting, which means inevitably they will end up coming from blogs, SFSE, Stack Overflow, GitHub, etc.
Unfortunately, what they have done in this case is most likely breaking the Creative Commons license, which requires attribution. Since this entire site is on CC, it's theoretically possible that SO could decide to go after salesforce.com for infringement. I seriously doubt this would happen, because it would be bad business, but this brings up an interesting point: the KB articles should be attributed to wherever they came from.
If these were published papers in academia, they'd be in trouble for plagiarism immediately, because these are obviously direct copies with minimal cleanup and zero attribution. While I can't say that it ever will cause problems, I can say that, since it's been observed (actually, I've known this for years, but just kind of ignored it), they probably should revise their practices before someone who does care about such things gets their content lifted without permission.
I wouldn't even know who to talk to about this one, but you might want to ask around and see. It wouldn't be that hard to add a custom field that includes a URL to the original source material (this is Salesforce!), although unfortunately, all currently published material would probably never get attributed, since it'd take ages to Google and attribute all the original source material.