Enterprises usually own their servers, so I would grant that caveat, but even that is going away with cloud providers.
Of course being user owned, by definition users should be allowed to outsource their email service to large companies, and they do, because it's a lot cheaper that way.
But an email server they own themselves, with their own rules, own display preferences, compatible with billions of other email accounts, is well within their reach. And millions of people do just that.
The distinction is that 99.9% of consumers and an increasing number of organizations don't want to do it in house and would rather let someone else do it.
Again, it's not about capability, it's about what people actually are doing.
You couldn't build your own version of fb that can view posts from your friends.
You can setup your own email service to view emails from your friends who are using @gmail.
That's an important distinction.
Try sending a tweet to a Weibo user and see how that works out.
In the name of fighting spam, Google, Yahoo, AOL, etc. have made it a real chore to get messages delivered to their networks. I still haven't figured out how to get messages delivered to AOL.
Usage defines the capability. Just like anything.
Twitter (and other dominant free products like Facebook) will behave the way our overlords want them to behave. Consumers have surrendered the free market. Capitalism is only played behind the scenes.
It can absolutely work. The product is not free. The product is our attention and internet services (Twitter/Google/Facebook) are competing for that limited supply of attention in order to sell it on to their advertising customers. We won't give our attention to them for free though, so they have to buy it by providing a service we want. If they don't provide an attractive enough service, we turn around and sell our attention to someone else. That's how this market works.
Perhaps the problem arises when the product is not only free but also a virtual monopoly. Changing the TV channel is easy. Changing to a Twitter alternative is not. As consumers, we get to choose between an 80% Twitter or no Twitter at all.
You can claim the market will solve this (users will sell their time and speech to a higher bidder, now that Twitter's prices no longer seem competitive), but if people were empowered to have their computers do what they want, it never would have happened in the first place.