I'm positive the second comma shouldn't be there. (I wouldn't normally comment on commas, but it seems on-topic...)
I would have written "Writing clear and concise sentences"
Writing clear sentences.
and then 'concise' was added. As if to mean: Writing clear (concise) sentences.The "concise", like the "clear", in "Writing clear concise sentences" is a restrictive adjective, primary to the meaning of the sentence, with all words spoken at the same pitch. Clarity and conciseness are orthogonal. Perhaps there's a slight pause between clear and concise, which could be shown with a single comma.
http://en.wikipedia.org/wiki/Serial_comma
EDIT: Ignore this. I thought he was referring to the first line of the post. "Follow the principles below to write sentences that are clear, to the point, and easier to read."