My guess is that they wouldn't be teaching if they didn't have a dan grade under their belt. Seems like your anger/frustration should be directed more to the those who promoted them in the first place.
Can you really blame the sandan who sucks for being deluded when some shihan somewhere gave them that rank and told them they were ready to teach in the first place?
I am simply not in a position to comment about what goes on "above my station", especially on the internet. Suffice it to repeat what my own teacher Saotome Sensei said, if the student doesn't do well, it's the teacher's fault. One can easily extend that to instructors who are below par.