SmsTransport checks only that SMS are not completely disabled (all checkboxes unchecked), but does not know, if the current SMS is a fallback or just a normal SMS. Actually there is a bug in the current version, that it is possible to send SMS to non Textsecure Users, also if that option is unchecked, but the SMS fallback is allowed.

However I also would prefer to do these checks in the SmsTransport deliver method. I just did not know, how to check if the current SMS is a fallback or just a normal SMS to a non Textsecure user.