It may be hard coded into their encrypted closed source files, I have not been able to find anywhere in the psa database where they have those values defaulted..... I've searched for it a number of times (as well as other default values).
Once the domain is created, it creates entries in database: psa table: Parameters parameter: nonexist_mail value: reject
Table: misc
Param: mail_bounce
Value: true
If this value is changed to 'false' then the 'bounce' option will not show up under Mail preferences.... but it also does not select either of the remaining options by default then.
Usually I go through all options and settings when creating a domain just to make sure that it's the way I want it.