To be a licensed plumber you must know how to work in all areas of plumbing.   However tankless water heaters are the new kid on the block.  So training has not been incorporated into the plumber training process yet.  There are some plumbers who decline to work on them, because tankless water heaters are highly sophisticated electronic gizmos and they are just intimidated.   I attended a class in Griffin Ga. at the Rinnai plant so that I can work on tankless heaters with confidence.