SGSK was created before NGUI was. In fact, the SGSK build up there won't work with NGUI, which is why I offer alternate versions of that kit -- one with NGUI integration, and another with NGUI + TNet.
There are plenty of assets up there that use NGUI, but as Baroni mentioned, the best way to go is to not include the NGUI folder when exporting the package for the Asset Store.