Santa comes from a Northern European tradition. Of course, he's white, just like Odin, Thor, and King Arthur.
Would we have a black kid dress up like Odin, Thor, or King Arthur?
Why not?
Indeed, there's nothing stopping anyone, of any colour, from dressing-up as any of those listed above, However, problems arise when it appears that reality is being deliberately distorted to present an innacurate picture to the audience, as well as being deemed disrespectful.
I think it's ridiculous for film makers to cast a black man to play a role which portrays a character everyone with an ounce of common sense kows was white, especially in a historical context. The same as a white character playing an African slave, which would be both dishonest and ultimately hurtful. Like I said, it cuts both ways.