Absolutely, because the people making the clay are catering to art classes and schools, none of which are likely to care or have a real obligation to provide clay that can become vitreous and withstand outdoor use. So the Clay Companies have absolutely no obligation to provide a bonsai Potter, a mere -1% of the potter population, with clay that is good for our uses. They have absolutely no $ to be had from providing clays for outdoor use.
The closest you get is Sculptural clay, I reckon because "sculpture" sounds big, and big, like our trees, fits outside.
That is the reasoning. Legit. Stupid.
The "out" of any liability is this, words spoken from Tom Turner, famous for his Porcelain and Porcelain works for decades, "They don't mine clays, or pottery chemicals, for potters. It's made for industries much larger than pottery."
When speaking of raw materials. Ones which a potter may use to create his own "clay body". Not shit in a bag.
Red Clay, the Grollegs China or English white Clays, Fire Clays, yellow clays.
Glaze chemicals; Opacifiers, colorants, flint, alumina.
Everything is made for other industries, from cosmetics to food.
So not only is there no obligation to the Bonsai Potter who, for all intents and purposes within the clay industry, doesn't even exist.
There is not even an Obligation to potters at all!
So trusting store bought clay is unreliable.
"Bag clay". Fuck it.
You best test Mine!
.........
I'm bought to hit my River, and clean more a this red and yella. Work it and test it till it's right.
That's a clay body.
Bag Clay. Shiiiiiit.
Sorce