I don't see why it 'exploits the common good' to want to charge or monetise a service you provide.
To me this is a breach of trust - i.e. the underdogs are all about openness, they're growing based on the goodwill of the early adopters, then once they get big, they forget about openness. Personally I couldn't give a damn about Twitter's need to monetize, they should have thought about that since the beginning, before promising things they couldn't deliver and growing based on the generated trust.
Twitter is not alone in this, there are many other offenders, including Apple and Google. And they should really pay attention to what happened in the past, because once you piss off a significant portion of the industry, you can only go downhill from there. Companies like Microsoft can testify for that and oh look, they are the underdogs again.
To charge your users, no, of course not.
To charge advertisers and make your users products instead of customers, that's where the problem is.
(And part of the problem is that users have become so conditioned to getting services like Twitter and Facebook and Google for free that the idea of having to pay for those services would shock them.)
There already are Twitter apps for phones that don't display advertising.
(Previously I'd been scraping together Yahoo Pipes or Google App Engine based things people had made, each of which would work for about six months before something finicky would break 'em...)