Simple people have a hard time with complex issues. This is a complex issue. Some schools have strong connections with some tribes. This is the case with FSU, who have the endorsement of both the Florida and Oklahoma Seminole Nations for use of their likeness.
This is also the case with Central Michigan and the Chippewa tribe.
Basically it comes down to a tribe or people having rights over use of their likeness or cultural identity, which also means the right to use this for financial profit. Yunno, like Iowa's claim against Southern Miss with the Tiger Hawk / Golden Eagle issue?
The culturally offensive part is about a group of individuals in power co-opting the cultural likeness of a less powerful group without the endorsement of that group. Yunno, kind of like when these groups were forced off their land without permission?
So what's the problem with the Sioux? Well the different factions of the Sioux have not come to an agreement about endorsing the use of their likeness. They have rights over that likeness, just like Iowa has rights over the Tiger Hawk.
But I am sure I lost some of you after "simple."