He didn't call himself Toby. The shirt says Kunta Kinte signifying that like the character in her roots, he would never bow to masters. Kunta would never sell out. He would always fight for what's right....freedom and liberty.
Thats why Kap wore the shirt. He's not trying to compare himself to a slave. In Kap's mind the NFL, Trump, and millions of people's insistence that he stop protesting police brutality during the national anthem was analogous to them wanting him to be Toby. White men in this country have ALWAYS tried to dictate what, how the black man should exist in this country. But men like, Malcolm X, Dr. King, and Ali have resisted the white man's attempt to be "Toby."
Kap, keeping with the example these men led, is saying, " I will never call myself Toby. You do not get to dictate who I am and what I do. I'm Kunta Kinte. I was born Kunta and I will always be Kunta Kinte. You cant break me or make me. I am who I am. "