I do have a canister of unsweetened cocoa powder in my house, and it contains just that: cocoa powder. No lactose, no sugar. I think the reason chocolate isn't allowed is as a previous person already said, even without the sugar and other stuff often associated with chocolate, the cocoa itself has addictive tendencies for some people (though I have never especially liked chocolate, so I guess not addictive to everyone).
I had been on the SCD for several months, and then ate a dark chocolate covered almond. One (1). Someone gave it to me, it seemed like a good idea at the time, and I ate it. Pretty tasty, but not worth the 3 hours of very bloated gassiness it gave me later that day. Wow! That experience alone made me not want to eat any more chocolate.