I don't understand your problems with them. If somebody wanted to do art, but an experience made them change their mind, why shouldn't they be allowed to have a foundation year, and then go up. If they went back to college as you said, then they would have their science A-Levels, and would be directly challenging your place at medical school, whereas with these foundation courses, places are previously alloted to these people. Many people do actually come from disadvantaged backgrounds. Just because one person you know didn't, doesn't mean that other people don't. Everybody deserves an equal chance to get into medical school, and so someone who went to the worst school in the country won't have the grades, or possibly was never pushed to work.
Personally I think you just have a problem with it because somebody you didn't like got into a foundation course, and you either found it hard getting in, didn't get in, or are just on such a high horse that you believe someone who wants to do medicine should have wanted to since they were in the womb.