Not trolling, but rather a good question. I've wondered that myself.
Since the shield is truly unique -- one-of-a-kind -- one could imbue it with any property one wants, and who could question it? It could be argued that the almost magical properties of the shield are due to the vibranium/element X alloy it's made from. At one point, it was said to be made of an adamantium/vibranium alloy, but this was later refuted. Now, we don't really know what the other material is that makes up the shield.
Therefore, I will theorize that the shape of the shield is key: When a force, like a bullet or the Hulk's fist, hits the convex surface of the shield, it absorbs almost all the kinetic energy. However, when the shield hits something edge-on, due to the way the molecules are arranged, it almost has the opposite effect, losing very little momentum due to impacting a wall or a Hydra goon's skull-bone, and is able to ricochet all around their subterranean hideout.
Still, your point about it being used as an offensive weapon is a good one. Why, when Cap slams the convex side against Batroc's chest, doesn't it just feel to Batroc as an almost insignificant tap? Ya got me.
I had also considered the edge on thought, with a similar thought as you that the way things are aligned that when hitting on the edge all energy is transferred(or at least not absorbed), but as you said as well... any normal shield punch with the front of the shield should still be a love tap at best.
In fact i suppose if cap charged into someone leading with the shield, it should pretty much stop him dead in his tracks upon impact by absorbing his forward force/energy.
hmmm, i shall ponder a bit more.