What you are probably seeing is an INSIDE corner vs. an OUTSIDE corner. Yes an outside corner needs to be metal due to thing hitting it, you dont want it to easily break. An inside corner needs only to be taped because its highly unlikely that when you are hauling in furniture that the leg of a couch is going to strike an inside corner. Hopefully this is the case, tape on an outside corner would be incorrect.
Thanks for the question, Andy Love Ask the Expert.