It's 2014. Passwords need to be treated as a serious matter. Legacy system or not, there is no good reason to reduce keyspace. As others have mentioned, this is a sign that passwords are being made compatible with old phones without these letters available... and likely the passwords being reduced to numbers before storage. The prohibition of special characters also seems to corroborate this.

Furthermore, the poster below who showed screenshots of being emailed their password indicates JetBlue is storing passwords either in plain-text, or encrypting them (both just as bad), instead of properly cryptographically hashing before storage.