Universities tend to look at your situation, school etc. during admission. They want to see that you’ve excelled to the maximum given the situation that you’re in. For different ethnicities, this sometimes comes with different socioeconomic and educational opportunities generally speaking.
In theory, it should be just as ‘hard’ for any applicant to be accepted. The theory is that privileged applicants have more support (and research generally proves this to be true) so the statistics could be interpreted to mean that it is easy for minorities (with the exception of Asian/Indian) to be accepted. Universities clearly believe that it is equally difficult for a minority student to achieve to perhaps a slightly lower level than a European/Asian applicant, and hence as an Asian/European applicant, the statistical bar required is slightly higher.
I don’t think it’s fair to say that ethnicity is valued over ‘actual merit’. Actual merit, to me anyway, implies how the student has been able to take advantage of their situation. A student who’s had no academic support and scores 1480 on the SAT, may thrive more than a student who has always had support and scored 1550 for example - especially when opportunities are evened out at University.
Affirmative action isn’t perfect, but it does a good job of:
a) ensuring diversity at University
b) helping & rewarding those who have made the most of their sub-optimal situations
In regards to the MIT part of your question, I’m not an expert however my take on it is that MIT is first and foremost a US University. The purpose of having foreign students is money related, reputation related, but also to help their domestic students be in a more stimulating environment. For a domestic student, there may also be some degree of duty to educate the brightest Americans.
Hope this helps a bit.