Twitter has done a Google by switching to HTTPS by default for all users.
Users can still disable HTTPS on their accounts by going to settings if they have a good reason to do so.
The company said:
Now, HTTPS will be on by default for all users, whenever you sign in to Twitter.com. If you prefer not use it, you can turn it off on your Account Settings page. HTTPS is one of the best ways to keep your account safe and it will only get better as we continue to improve HTTPS support on our web and mobile clients.
Twitter has offered HTTPS for a while now though it had to be enabled manually from the settings.