This is going to be a bit long-winded, so grab some popcorn and Coca-cola, and relax.
An article from 24fightingchickens.com gave me the idea for comparison purposes.
Let's compare the aspiring martial artist who wants to eventually be a top-notch karate-ka, to someone who wants to become a professor.
Essentially, the black belt is like a college degree. If someone graduates from the Massachusetts Institute of Technology, then you can assume that the graduate is reasonably proficient in the knowledge that was conferred upon him. In order to do that, he had to at least have fair work ethics (study, research), and had to meet certain standards in order to graduate.
The same, however, cannot be said about that very same person when he was a freshman in college. Why? Some freshman fail out, some leave on their own will because they don't like it, while others would rather be doing something else. The freshman is a relatively unknown quality at this point, and only through time and work, will his true abilities shine (or not).
As that college student progresses through the years, and manages to do well enough in his coursework (decent GPA, etc), he becomes more of a known quality, and eventually, one can get a pretty decent idea of how he will turn out.
Once he graduates, he is now ready to move on to more advanced studies, should he decide to do so. This way, if he did well enough, he'll get his chance at progressing through graduate school, and can move on to bigger and better things.
Once he gets his advanced degree (let's compare a master's degree to a 2nd dan, and a doctoral degree to a 3rd dan), he could very well be ready to take a professorship, but if he wants to do this at a more prestigious school, he will need further training as a postdoctoral fellow. Also, the person that he trains under is going to have a pretty big impact on how he is viewed by everyone else. It's no secret that working for certain big name professors will get you more publications in better journals, etc. While he's a postdoctoral fellow, he may actually get some teaching duties that a professor normally would.
If he does well as a postdoctoral fellow (let's compare him to a 4th dan), then he has a chance of finding the professorship that he wanted in the first place.
Now, let's say that he does become an assistant professor (lowest ranked tenure track professor, let's compare him to a 5th dan). From here on out, there's no more "testing" going on; rather, the school that took him in will be evaluating his overall achievements, over a period of time. Once they deem that he has brought enough to the table, he'll be given a chance to become an associate professor (6th dan), and further down the road, a full professor 7th dan and beyond). Such promotions can only be done through a committee of full professors who must generally agree that he is a candidate for such elevation.
Does this mean that someone has to go through all of that? Of course not. If one's aspirations aren't nearly that lofty, then he can certainly become a professor at a less prestigious place, even with a Master's degree, and if he's a good teacher, then he'll be producing potential graduates as well. Just because someone "only" has a M.S. doesn't mean that he'll be an inferior professor.
Now, let's compare this to someone who 'attends' a junk college. Let's use one of the highly unscrupulous 'online degree' programs that are nothing more than some guy (let's compare this to, say, our favorite neo-ninja named "Radford") in a warehouse that mails out fake transcripts. Someone who graduates from this 'college' is going to have a hard time getting into graduate school. No decent graduate school is going to want to accept him, knowing that he came from a garbage school, and may, in fact, be worried about his character!
Note: I am NOT inferring that a 2nd dan is the equivalent of a MS or a 3rd dan is the equivalent of a 3rd dan; I'm just using these as easy to understand reference points that may, or may not, have any validity. This is just something to give a comparison, nothing more.