Because they were quite angry with his unethical behavior, especially how proud he was that he “played the system”.
That may be true, but while the Indian communities in East Africa were there, most didn’t seem to integrate or have interactions which left positive impressions with the indigenous African population.
This was underscored when my cousin was newly married and honeymooning with wife in East Africa, they were perturbed by the level of vitriol openly directed against “Asians” in the conversation by their East African hosts. It was only after one of the hosts noticed their nervousness that they clarified that in their East African nation, “Asian” referred almost exclusively to those from the Indian subcontinent and the vitriol was due to decades of history of frictions between East African Indian communities and indigenous Africans as they were perceived by the latter as too insular, disdainful of, and quite high-handed/unethical in business dealings with them.
Historical factors which in retrospect might explain the level of anger from the African/African-American classmates who were among those who confronted that unethical classmate of Indian descent who marked himself down as “African-American” on his college apps.