Why would you want a pastebin app to encrypt your stuff for you? A pastebin is a place where you just post something for random folks to view. It's like imgur for text. Yeah, there's ezcrypt, but I honestly don't see the point.
If you want to password-protect your paste, you are free to pass it through GPG before posting it. Much better security than trusting a third party's JS library which may have been modified in transit or even contain a back door.
By encrypting the data before turning into a URL neither the server, nor anyone who obtains that URL would be able to read the message without knowing the password.