If they just quietly gave an ambiguous non-disparaging statement like "we're forking because we're unhappy with the direction Docker is taking", it would seem frivolous and ill-considered, and nobody would know on what points the fork would be aiming to distinguish itself.
This statement needs to be made, the way it was made, for the same reasons any project announcement is made: it needs to announce that it exists, and why it exists. It's the same as Docker's "debut" blog post(s).
Every schism needs its 95 Theses, and the odds favor the ones who can read them, understand them, and take them into consideration.
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Disclaimer (re https://twitter.com/kenperkins/status/539528757711622145): I make edits to my comments after posting, usually posting a line or two then fleshing them out over time. If I make a change that conflicts with a statement in an earlier revision, I'll note it: otherwise I'm pretty much just composing live.