Dual licensing with a copyleft license is common if you want to offer an ability for someone to develop a closed-source project: they can perfectly develop a GPL-licensed commercial project without paying anything.

If CC-BY-NC prohibits commercial use, it is not an open source or free software license (which have compatible definitions, but differ in motivations).

AFAIK, Creative Commons was set to create a set of licenses in the spirit of open source for creative works, and I wouldn't expect them to be open source at all.