Why did Twitter make some tweets invisible?

Twitter users, here’s the obscure but possibly important "small settings update" the company may have made to your account’s configuration: If, under the Notifications panel on the Settings page, you had changed the default setting to either "all @ replies" or "no @ replies," your setting has been reset back to the default: Replies sent from people you follow to people you don’t won’t appear in your public timeline.

Twitter co-founder Biz Stone says on the company’s official blog that the change was a response to user behavior . But Download Squad writer Jay Hathaway thinks the company was worried about an increasing number of one-way replies to celebrities who don’t reply back. This, Hathaway suspects, make Twitter look like a home for unread fan mail rather than an active ecosystem of two-way, three-way and more-way conversations.

I think Hathaway is probably right. If Twitter only wanted to change the default for most users, why didn’t it leave "Show me all @ replies" as an option?