Quick question with wiring running on the wall of an unfinished basement... looking for code clarification.

Code states for NM:
334.15(C) says, in part, "NM cable used on a wall of an unfinished basement shall be permitted to be installed in a listed conduit or tubing."

Now, as the sentence reads, it says "shall be permitted"... The what the rest of the NEC is written, this grammar seems to suggest it as an option rather than a requirement. I know wire run down the wall (for example - to a washing machine outlet) must be protected from physical damage... But I can't find anything that requires it to be armored or in conduit. Since 12ga wire is permitted perpendicular across joists when stapled to a running board... does the same situation apply on the wall? i.e. Would a running board suffice, or must the wire transition to armored/conduit

The way the NEC appears to state the issue is that you can put the wire on running boards but "it is permitted (allowed)to be in conduit". There have always been people raising an issue as to whether NM is allowed to be run in conduit and at least for this purpose, it is permitted. I have seen it installed in brand new non-occupied houses, in conduit, running down the inside basement wall to the GFCI outlet.