hard wiring a wall unit a/c
Nothing weird about that.  Being hardwired, most likely it's a 204V air conditioner.  

Wire colors are to distinguish wires.

Sometimes two wires are just instances of the same thing.  If you swap the wires, everything works.  


the two "Messengers" in 3-way switch circuits 
2/3 phases in NYC or Euro style mains power, e.g. 208V or 388V to an A/C unit
The two "hot" poles in 240V wiring of split-phase North American power 


The wires are interchangeable and it's useless to distinguish them from each other.  But very useful to distinguish them from other wires with other duties.  Therefore, no reason to waste a color.  

However, 120V line cords are sometimes ribbed

That said, cheaper line cords, e.g. used on 120V lamps, often have no inner colored insulation, just an outer sheath that can be split.   They distinguish sides with one side of the cord being ribbed. The ribbed side is neutral.   However in the above cases, it makes no difference.  

Why do you expect them to be red-black?

Because of the stock colors found in multi-conductor cable.  They make each one a different color because they have no idea what your use will be. 


/2: Bare Black White
/3: Bare Black White Red
/4: Bare Black White Red Blue


That makes sense. Why stock several color combos of /3, when we can just tape the wires to mark them different colors? 

For instance, white/gray=neutral (as well as green/bare=ground) are the only mandatory colors. Other than that, anything goes. In house wiring, the convention of Black=always-hot and red=switched-hot works reasonably well for simple boxes.  But on a complex box it can get out of hand really quick, and that's when you want to do things like tape the messengers yellow, tape the second hot and neutral brown/gray, etc.