I think there's a certain ego thing going on in the question "How dare someone call themselves a professional when they're not as good as I am at <insert skill here>"
Being a DM (in PADI, not NAUI), means you have the skills to recognize another diver having problems and help in some way, can lead a dive, can assist in a class. Basically, you're at the level of a student teacher in school.
Do you go to your children's schools, notice the college junior helping with the class and ask "How dare this kid call themselves a teacher, they don't know nearly as much as I do?!"
Well, then, if it bothers you, belly up to the damn bar, get your professional certifications, and become part of the solution to the problem you see.
New DMs are people who are starting down the path of becoming a diver professional. Like all professions, there is a starting level where people learn the basics of doing that skill. (which is why I won't let interns or residents perform medical procedures on me, but that's another discussion

) And that can take a long time for some folks. Some people will never go beyond that point.
We all agree that the most important aspect of any class is the instructor, but we don't want a large pool of people competing to fill the next instructor slot becuase some of them don't have the skill level we believe (usually incorrectly) is necessary for their current level?
Personally, I wish every diver would pursue the DM certification as soon as they feel they're ready. Doing so would raise the level of diving in general and would give an even bigger pool of people to pursue instructor ratings.