Scholarships can use whatever criteria they want: ethnicity, gender, age, intended major, artistic talents, etc. There is even a scholarship for tall people and another one for those with red hair.
Minority scholarships exist to entice those minorities to go to a particular school. Some schools want to increase diversity; to attract the minorities, the school offers the minority students money. Other schools and organizations want to make sure minorities have the same educational opportunities as their White counterparts. It's the same principle as an athletic scholarship.
And there are plenty of scholarships that white people qualify for, even those "minority" scholarships. You won't find a "white only" scholarship, but you can find ones that don't look at ethnicity. And there are minority scholarships are actually intended for all students, they just focus on attracting the minority students more.