Why is it wrong to use things in a way that is contrary to their designed purpose?

It makes them less what they are.

you mean if I stand on a chair it becomes less of a something to sit on?
Exterior Utility affects the essence of a thing?

I'm not sure what your second question means, but the first is definitely along the correct lines.

Although it's not a perfect example, since using a chair to stand in is not contrary to its purpose.

What would be an example of using a chair contrary to its purpose? Just NOT sitting on it?

My previous "second question" might be better asked this way: Does every thing become less of what it is when it is used in a way contrary to its purpose? If so, does it become more of nothing or more of the something it is being used for?

I'm having a hard time thinking of how you'd use a chair contrary to its purpose. Not sitting on it is not using it, so you're not "using it contrary to its purpose."

To the second: In the case of things like chairs, as long as the chair isn't damaged, it will continue to be a chair even if used (somehow) contrary to its purpose. But every time a human uses a human faculty contrary to its purpose, he diminishes his use of reason. Since reason is our specific difference from all other animals, misusing it or misusing things under its direction makes us less human and more dumb animal.

Is this the standard explanation? I prefer the explanation that every time something is used against its final cause it become less of itself, because IMO it's really is the reason why [ie. why using something contrary to its final cause is bad].

I do see the reasoning that using something contrary to its final cause diminish reason. But IMO it's not the ultimate answer. It's not the "right on" answer. The ultimate and "right on" answer is that using something contrary to it's final cause will make that thing less of itself (to the point of becoming a nothing).

There is nothing morally wrong with using an artifact contrary to its purpose (assuming one can even figure how to do such a thing).

But there are more reasonable ways of using things, more elegant ways to use reason.

Sitting on a chair makes more sense than standing on one....

Edited to add:
It makes your use less reasonable to use a chair for standing than sitting.
Because of the sacred nature of the soul, it is more unreasonable, more inhumane to use people contrary to God's will.....

I'm not sure it is necessarily less reasonable to use a chair. If the chair is sturdy enough and tall enough, and if a real step ladder is not readily available, then it seems to me that standing on the chair is in fact the reasonable thing to do.

Obi, correct me if I'm wrong, but chairs don't have final causality in the same sense that true substances (e.g., persons) do. As artifacts or property-things, their parts are ontologically prior to their wholes (unlike substances). The same would hold true with their ultimate capacities.

So, it would seem to me, to generalize, that the misuse of artifacts is essentially different than the misuse of substances, in that the final causality of the former is part what the thing is in and of itself, whereas in the latter, the final causality of the thing is rooted in an artificial, conceptual unity.

It's not a full answer, but I think that gets us going in the right direction about the difference in artifacts and substances and their "proper" and "improper" usage.

Right. The chair is a good stepping stone (sorry ) towards an understanding of the basic idea, but it doesn't get us all the way there.

What is the purpose of a chair? When we talk about purpose, we at least mean what the designer had in mind, right? Are there are things to consider when nailing down the purpose of an artifact?

Note: I am fundamentally unfamiliar with what makes an artifact an artifact, and a true substance a true substance. The idea of final causality also mostly escapes me.

Substances are natural. Artifacts are man-made. The latter are more commonly talked about in modern philosophy as property-things (although, as in all things philosophy, don't take the two terms as having a 1:1 correspondence!). Final causality is best understood in a broader discussion on the four kinds of causes generally (efficient, material, formal, and final), and I found it particularly helpful to see how it relates to the actuality/potentiality distinction (where actuality refers to how something is, and potentiality refers to how something could be). On that view, efficient causality is the actualization of something's potentiality; that that means the efficient cause was directed toward that potential, which is to say, its final cause.

The Catholic Encyclopedia has a good article on the subject of causality (including final causality).

trees are natural.
chairs are man made?

a chair is still tree, or do we only talk about things in terms of their most recent known purpose?

Obi,
The point I want to focus on is this:

What is the difference (if any) between using artifacts contrary to their purposes and using true substances contrary to their purposes?

How CAN someone do either of those things?

A chair is not a tree, unless it's carved into living wood. Wooden chairs are made of material that once formed the material of a tree, but if it's no longer alive, it's no longer a tree.

Your questions to me are more complicated and I have to do a few other things right now.