That's not exactly true. If you look at the methodology of the CBS poll, they reweighed to get to their sample. The sample they got was slightly more Republican before they settled on a D+5
http://www.cbsnews.com/news/hillary-clinton-donald-trump-tied-going-into-conventions-cbsnyt-poll/(If you scroll down to the bottom of the Scribd document, it clearly shows how they reweighed the sample. As always, the devil is in the details.)
I think the Marist state polls begin with a D/R/I sample from either the SoS or exit poll numbers from 2012 cross-referencing the voter rolls and get their reconstituted self-identifieds from there in the states that have registered D and Rs.
The rest of these pollsters do not, unless otherwise stated.