I completely agree with you in that this is currently a weakness in the system, which can be exploited by those savvy enough to realize that they can gain a pretty large advantage simply by putting in a relatively small amount of dedicated work.
Having said that, if (as many of us believe) these sorts of interviews aren't a very good way to correlate candidates with desired job skills, then the situation is bad for companies, and for the efficiency of the industry as a whole, despite being a good thing for that set of people who are willing to gain an advantage through preparation. There are probably more useful things than preparing for interviews that people could be doing for that couple hours a day.
Having said that, if (as many of us believe) these sorts of interviews aren't a very good way to correlate candidates with desired job skills, then the situation is bad for companies, and for the efficiency of the industry as a whole, despite being a good thing for that set of people who are willing to gain an advantage through preparation. There are probably more useful things than preparing for interviews that people could be doing for that couple hours a day.