First nations laws are intended to apply on first nations lands, and first nations can make their own laws. Having the force of law is not needed. Basically, the force of law is intended to clothe first nations laws with the strength of federal laws. Because this applies on first nations lands, in clause 6, Parliament is saying first nations have an “inherent right” to make laws.
It would seem a bit contradictory to say that in order for those laws to actually have force, you need the force of federal law. They should be able to apply on their own force on first nations lands. This provision would seem to be inconsistent with paragraph 6(1)(b) and the recognition of inherent right.