I have puzzled on it too. I have never seen mention of rules that said that I could not repost material from my other websites, but maybe I missed something. I don't know why that might be against the rules, after all I am the original author and publisher, right.
Most of the time I have added new photos or other material, edited, and tweaked the original work before posting it on Steemit. Sometimes the new post is not very much the same at all, but the bot picks it up anyway.
Not much of a problem for me in the future though. I'm about all out of older material.
Time to forge ahead, and get the Steem on!
Best!