whilst the notion of charging is often mooted there are very good reasons for this not being an option - not least, it was tried right at the very start with spectacular failure. It comes up from time to time, but typically because people aren't aware that it was tried e.g. heres it popping up 4 years ago with reference to the original abortive efforts http://forums.asp.net/p/948570/1157402.aspx
As that post mentions we also tried to make financial support optional, but with some additional benefits i.e. access to private forum monitored by core team, avatar, dotnetnuke "adsense" style adverts - you can read them @ http://www.dotnetnuke.com/tabid/938/default.aspx . Whilst the revenue that came in from that program was vital to keep the infrastructure running, it was still small with only a small portion of people paying (there was an additional benefactors program which also had limited success). Whilst we'd all like to think the best of people and assume that anyone using dotnetnuke (or even those using it in a business capacity) would be willing to pay something towards it's survival, time and again this was proven to not be true - and this is the case with any other opensource project I've seen.
This reality often means that projects collapse under their own weight, as money is too tight and things stop getting done - it's easy to find a volunteer to write a cool new feature, not quite as easy to find people willing to upgrade servers at 3am on a sunday night. As such many succesful projects create a commercial entity and try to generate money, and use that to underwrite the contined success of the OS project. How they choose to do this is up to them, and their company will rise or fail based on their business plan (which trust me, will change quickly if the company starts to fail).
I'm also continually surprised when people list other OSS projects and seem to suggest that they get by without funding/commercial entities - if you do some googling you'll find most of the bignames also have vc funding or else sponsors with very deep pockets
Wordpress - hosting is interesting, but is typical a low margin, high volume case. As wordpress runs on linux using mysql, they have limited capital costs so can afford to giveaway hosting and make money on upsells . This is not an option for an asp.net CMS that requires windows and sql server licenceing. (Note: the $29.5 million they raised from VC's helps buy those servers - http://gigaom.com/2008/01/22/wordpresscom-creator-raises-29m/)
Drupal - their commerical entity , acquia, raised $7million (http://raincitystudios.com/blog/acquia,-newest-drupal-startup,-announces-$7m-funding) - currently they provide support , network services and additional documentation similar to how PE does.
Cathal