Bob, although most carpeting is stitched in straight rows, there is a stitching method called a zig-zag stitch, which is likely what you have; not truly a diagonal, but it does "zig-zag." Most mills have gotten away from using this method because it does carry an inherent problem with visible seaming. Carpet layers tend to hate the stuff, too, because of the difficulty in seaming. The tufting technically should still wind up in rows, but it often doesn't quite align perfectly, thus the problem.
I'd honestly forgotten there even was such a thing, but contacted my uncle (a carpet broker with over 30 years production/sales/installation experience) to double-check on this.
Cut-pile carpeting is knife-cut with a straightedge from the backside; berber is cut from the face with a row cutter, a special knife which has a long guide that travels along the backing parting the way (like walking a row in a cornfield) & forcing the blade to travel in a straight line between the loops. If the loops don't align, this would be a very tough material to make look right, so the salesperson is basically correct & not really trying to "pass the buck."
I'd give the mill rep a chance; let him look at the situation & hear his thoughts before making any demands. If he tells you that this is the best that could be expected and is going to leave it at that, then I'd bring up the fact that the mills know about the seaming trouble, and that in not warning about it in some manner you feel they should be responsible for correcting the matter, whether that means having it re-done in the same material or a similar one.
Most mill reps I've known were quite easy to get along with, and didn't shirk from doing what was needed to make things right for the customer. See what he/she says before going to battle unnecessarily. Good luck Bob!