I don't think the issue is that bands don't want to tweet. I think what becomes an issue is that being on a label comes with the agreement that they will take care of the bulk of your promotion. For a label to expect artists to tweet or invest themselves in web 2.0 on their own time with no financial incentive is, IMO, wrong.
Of course, any artist who is interested in building his fanbase should understand it's a web 2.0 world. But I can understand why a signed artist might put up resistance to a label that isn't willing to do their web 2.0 stuff for them as part of the promotion deal.
Understand that right now, the record labels are spinning everything internet-related as bad. "The internet is ruining music sales." "The internet is ruining record labels." "Woe is us." Horseshit.
(You guys see the tweet about my free new song?)