As said, there is no source for what is written on wikipedia, actually they added the "citation needed" tag to it, so the info is not certain at all.
And the difference is, in English Uma does not mean horse and it is a name, so there is no reason to stick to the Japanese changes imho. And since there is proof that "Yuma" is their way of writting the name "Uma", to me that makes it a valid transcription and I do agree with the choice Viz made. But since there are no definite rules every translator bases their choices on different aspects.
But of course, you can always spell it Uma and pronouce Yuma
Personally I used to call Uma Thurman "Yuma" because I kind of assumed her name to be pronounced in an American/English way. Which might be another reason for her name to end up as Yuma in Katakana (though I don't believe they're as ignorant as me).