Fabricator added seam?

My run is less of the length of the slab. I was not expecting the slab to have a seam at all. He said he added it so the natural fissures in the black part doesn't have added stress to it. We're so deep in the process, that I'm kinda bamboozled. Did not mention of seam until today. A sink cutout is being added. Is this normal? Thoughts?