Since Firesheep tool was introduced last year, many web sites has enable HTTPS by default, starting by Google, Facebook, and now Twitter has also follows this good idea. They mentioned it on their own Twitter account. Please go to their Support Page for more information about this new feature.
Basically, this feature is not always turned on, so you must activate it on your preferences settings. Tick the "Always use HTTPS" option and press the Save button and you are done with it. It's just that simple
For sites that has HTTPS, but doesn't route you to their HTTPS URL by default, you can add it into HTTPS Everywhere URL list. Basically this is a Firefox extension that will force a website to load the HTTPS version if it's available.
Processing HTTPS is not as expensive or resource-hogging as before, so i guess it won't make big differences for the developers to add the HTTPS for the sake of their client's security and privacy.