Why is an encrypted message readable, when I view it in the sent folder in Mail.app?

Every encrypted mail you send, is being encrypted with the public key of the recipient. Using the according secret key in combination with the passphrase, it can then be decrypted by the recipient.

Since it is very likely, you, the sender of the mail, might want to have a look at your sent (and encrypted) mails, each encrypted mail will also be encrypted with your own public key. By doing so, it will later be possible, to decrypt the sent mail yourself.

If you go into your sent folder and immediately see the content of an encrypted mail, that behavior is not a bug, but a feature.

Please see the FAQ Article about storing passphrases to learn, how GPGMail gets access to your passphrase without you entering it.