What part of Africa Uhura was from wasn't the real issue back then, it was that Saldana's Uhura did not appear to have the same lineage as Nichelle's original Uhura. The complaint was that Saldana was cast in the role because she had a more, shall we say "commercial" look.
The problem a number of black people had with Saldana was simply as you tried to imply with "commercial" that she was too light skinned and her features not considered broad enough. It seems series and movie producers often prefer to cast black woman looking like Halle Berry/Zoe Saldana over some who look more like Viola Davis/Lupita Nyong'o. It is the same with Hispanic actresses. The "whiter" they look, the higher their job chances. So in short some think that it is colorism to cast a lighter skinned black woman in a role a darker skinned black woman previously played. Of course the Uhura/Zoe Saldana debate was nothing compared to when Zoe Saldana played Nina Simone. Now that was ugly!