Did you receive that email after registering? The controller action processing your request would have access to the POSTed plaintext password and could pass it right into the email template before it's sent (or queued to be sent). This doesn't mean they're storing it in plaintext.
If you request a password reset and they send back your plaintext password, then they likely are.
I got it over a year later, just cancelled a flight and got an email reminding me of the credit they set me up with. My login and password were in that email, so the above scenario does not seem like what is going on.
If you request a password reset and they send back your plaintext password, then they likely are.