It isn't technically "wrong". What it's doing, is trying to preserve the planarity of the faces; this may or may not be what your looking for.

One cheap way to do it, is to create a plane with as many divisions as you want, triangulate it, then sculpt it. It should then have all the edges facing the same way.

Just a quick question however. Why does the direction matter, and why do you need to triangulate the mesh? It's generally not done expect when working with old game engines.

By the way, excellent post. You've really covered your question well.