"The profession will determine the qualifications for employemnt."  This is the crux of what you're syaing here.  Now, if you're talking about certifications for having certain jobs (like you have to be a memeber of the bar association in order to practice law), then that's not true.  Those are state and federal laws, not ones set by the profession.  If you're talking about how certain employers won't hire you if you don't have a college degree, then that's valid.

However, the issue is that the NBA is a monopoly and, while the US courts have allowed sports leagues to maintain monopolies, the problem arises that there are no competitors for the NBA.  So the NBA is unilaterally telling every 18-year old in the country not only that they can't play in the NBA, but that they can't play in a major league basketball league in the United States.  The monopoly denies alternatives to the NBA for these 18 year olds. For instance, one company might not hire you without an undergraduate degree.  But every other relatively equivalent company still might.  That company's decision not to hire you doesn't necessarily leave you without any options.

Now, this doesn't mean I'm necessarily against the monopoly the NBA has on major basketball leagues in the United States.  I'm merely saying that because the US courts have construed antitrust laws to allow this monopoly, I believe that the NBA has a higher (non-legal) burden to not make arbitrary rules to exclude some people.

College is probably good for some people, bad for others.  The problem is that there is no other league in which to truly showcase your assets for the NBA other than the NCAA.  So it forces kids to go to college if they want to make the NBA, which is something the NBA should not be in the business of doing.

but if those bars get together and collectively decide that they're not going to hire certain types of people, that's collusion and it's illegal because it stifles economic competition.  the NBA gets around that because the age limit is part of the collective bargaining agreement with the players' union.

so like i said, an individual NBA franchise doesn't have to take on these risks of drafting 18 year olds if they don't want to.  and i believe when high school kids became big in the draft recently, many teams didn't take them because of the risks.  but as they saw what they were missing out on, those teams realized that the potential reward is worth the risk and so they took it.  that's the spirit of economic (and all types of) competition.  just because the NBA can do what it does within the bounds of the law (unlike other types of businesses) doesn't make it right.