Is there a place in the code that says you have to secure NMB cable to a metal box using a "romex" type of connector. From what I can remember as long as the wire is stapled within 8" of the box you did not need a keeper/secured to the box.
314.17(B). You have to attach cables to the box. Doesn't say which type of clamp to use only that the cable must be attached to the box.
Subsection (A) also indicates where conductors enter a box shall be closed in an appropriate manner. So having a big knock out and using a cable tie on the box would leave a rather large opening around the cable. Cable clamps cover the entire hole.