I think the problem people have with this casting goes into two areas:
Firstly, Roland is white in peoples minds, its that simple (I did see him as very tanned though, all those years in the desert an all). To be called or insinuated as being racist for observing that fact is deserving of ridicule.
Secondly, others (to a lesser extent than above) have observed a problem arising online, in university's, entertainment industry's and feminism especially of SJW's. These people, willing to accuse anyone who disagrees with them of racism/sexism/patriarchy and worse. This has mostly come from left wing political movements (before anyone from the left bites my head off, I'm not right wing, I'd rather take shrapnel to the nuts than vote Tory). There's a feeling that The Dark Tower Movie will become another victim of this fleeting hysteria, that the producers are somehow infected and allowing themselves to be effected by it. The gaming press has been heavily criticized and exposed for leaning to one side on the debate, and misrepresenting and slandering anyone who disagrees. They've even gone after peoples careers because people disagreed with them... I have a feeling this network will try to claim The Dark Tower, as they have done with various Marvel movies/comics to push their agenda further.
The only reason any of this bothers me in relation to The Dark Tower is, I want this movie to be a great. My expectations are probably to high in this age, but I'm thinking along the lines of Seven Samurai, Gone With The Wind, Lord of the Rings, Wizard of Oz and other classics. Hell even Harry Potter. (cue people making reference to changes made from book to film and how it didn't matter, mostly to people who didn't give a damn about the book probably)
For me, everything that needs to be in the film is in the book. Adding the political climate of our world currently, is a mistake (through subtle PC changes). Instead explore the ideas that are already there, and of the time of Roland, Eddie, Jake and Sue. Otherwise this film will only be "of its time", rather than the "timeless classic" Kings work in this case deserves to be.
To finish, I would rather The Dark Tower took some inspiration from The Hateful Eight, and not be afraid the explore the racial tensions/language that was used in those darker times of Sue's time. Because to observe them as they were written, gives her character so much more meaning. And shows the horror humans are capable of.