That’s because the endpoint device is supposed to have some manner of type B port (full sized, mini, micro) to denote that it is not a host device. Endpoint devices should not have a type A port for input, because double ended type A cables are not technically supposed to exist.
(Even though they self-evidently do, USB specs be damned. I have a flashlight that came with one, and its charging port is a full sized type A port. Using the cable it came with to plug two host devices together would be a bad idea and probably result in smoke.)
That’s because the endpoint device is supposed to have some manner of type B port (full sized, mini, micro) to denote that it is not a host device. Endpoint devices should not have a type A port for input, because double ended type A cables are not technically supposed to exist.
(Even though they self-evidently do, USB specs be damned. I have a flashlight that came with one, and its charging port is a full sized type A port. Using the cable it came with to plug two host devices together would be a bad idea and probably result in smoke.)