From here:
The following values may be specified in inheritsched:
PTHREAD_INHERIT_SCHED Threads that are created using attr inherit scheduling attributes from the creating thread; the scheduling attributes in attr are ignored.
PTHREAD_EXPLICIT_SCHED Threads that are created using attr take their scheduling attributes from the values specified by the attributes object.
The default setting of the inherit scheduler attribute in a newly initialized thread attributes object is PTHREAD_INHERIT_SCHED.
So, seems to be the same behavior as the one described in the question.
I would actually have thought that such a default value could be defined by the standard (without having that much experience with that though, I'll admit), but looking at the last line of the Thread Scheduling Attributes section:
(...) The default values of other scheduling attributes are implementation-defined.