Great question. Other more knowledgeable hubbers will have more to add, but I suspect that HubPages primary concern is whether or not off-HP sources will compromise online traffic, and consequently the revenue that can be made from your article.
Some of my favorite HP authors post content that they have published offline in print sources. I doubt HP or Google has the ability to detect that sort of thing, unless Google Books has indexed the work in print. Not sure what this means to your question, really, just my two cents to add!