i think a lot of it depends on how much attention is being spent focusing on doing DPS. This is the same reason that some multiboxers fail at DPS as well I think. in the case of someone playing multiple roles simultaneously as we do while multiboxing, especially mixed groups, it's easy to let concentration slip from maintaining an optimal DPS rotation. In the case of solo toons who don't perform I think often times you'd find that they're sitting there in /tells or watching TV/Movie or talking to people in RL or what have you...

Why DPS and not Tanks/Heals so much?
If a tank were to decide to play one handed while watching Dr Phil and texting his friends, you can bet that that tank wouldn't ever get to the point where you'd run into him in a heroic PuG (or the content has been on farm long enough that it just doesnt matter... but that's a different scenario). DPS on the otherhand can get away with that sort of behavior, and often do as far as I can tell.

Factors in DPS (IMO): player skill > Gear Level > Spec

As for what to do about said players... feedback is a good place to start, motivation to makes changes is important as well. We had a hunter join our 10-man uld crew, RL friend of the raid leader etc. He came in in a mix of T7/7.5 and struggled to break 2k. At first he was very defensive and offended that we questioned his performance. However, after pointing out that shamBot_03 who I'd been happy to replace with a real person was parsing significantly better DPS on the same encounters he changed the attitude a fair bit.

remedy:
He started reading up on how to play his toon, agreed to try a better spec and also agreed to turn off the TV during raid times.

edit: spelling and also to note that I don't pull top end DPS on any of my toons solo or multiboxing