The yum-builddep man page says:
The source RPM for the specified package must be
available in a Yum repository (which will be automatically enabled, if
it is disabled)
But adding an enabled source yum repository doesn't make yum-builddep function in RHEL 7. Editing the repo file to have enabled=0 instead of enabled=1 does make it work, though. Toggling it back to 1 again, makes it fail again.
It's apparently only looking at disabled repositories.
This request was resolved in Red Hat Enterprise Linux 7.0.
Contact your manager or support representative in case you have further questions about the request.