Twitter has apparently done away with one of its stranger formalities. In the past, users could only engage in direct messages (DMs) if they mutually followed one another. Not anymore. Twitter reportedly began rolling out a new feature for some users overnight, quietly adding the option to receive DMs from anyone who follows you. Those users now have a checkbox that allows them to toggle whether or not people they don't follow can DM them, with this prompt: "Any Twitter user that follows you will be able to send you a DM, regardless of whether you decide to follow them back."
If you're not seeing this option, it may not have been rolled out to your account just yet. Don't think, however, that Twitter's trying to mess with your privacy. It's important to remember that this is a feature to which you must opt in first. Meaning, you won't be getting spammed by stalkers unless you really want to.
As Mashable notes, this is probably less helpful for average users and more intended for businesses who deal with customer care issues through Twitter. Now they'll be able to carry on a DM conversation with a customer without following them back. (Mashable)