@Kellen Canfield it might be that vertical is in right now in that area. It happens, I see vertical all the time now. It is definitely something that needs to be on paper these days with all the hgtv shows changing what's normal.
It looks pretty good and will probably look even better once grouted. If you don't like it change it now before they grout.
Nicely saying I thought it would be horizontal is probably a good approach like mentioned above, then how hard is it to change it. I think if you want it changed just pay to get it changed. They'll have the cost to demo back out too and re prepping the thin set isn't going to be fun so it's not going to be a simple fix. Maybe he'll offer to eat the markup or something.
I remember having a brick fireplace painted and they painted it bright red, very odd looking, our normal spec was grey but we didn't do a full scope as it was just a minor rehab, paint and carpet.
Our fault totally and we paid to have it fixed even though no one would have ever used that color (I know that's a cheap mistake in comparison). Ive had the opposite happen where we had subway tile installed and the designer verbally asked for it vertical. It got installed horizontal even though the designer was annoyed I asked her if she wrote it into the spec and it wasn't so I didn't even say anything to the contractor. They probably would have changed it for me free, but I was enforcing some systems to prevent this stuff in the future and it helped everyone understand the importance of a written scope and spec. without upsetting anyone too bad.