Your comments show you know little of supplier/OEM relationships, OEM captive fluids, OEM fluid choices pertaining to market acceptance, and fluid choice in hydraulic systems as a whole.
You can "expect correct fluids" all you want, but you will get what the market in general wants to receive when they purchase the product.
Many OEM's specify their own captive part numbered fluids for use as hydraulic fluids in their systems. Those OEM's are also the ones responsible for warranty support to their own end user. The warranty given from the component supplier is 1 year against defects in material or workmanship (easily found on the website). Anything longer is 100% supported by the OEM.
OEM's choose fluids based on their market and end user acceptance/ease of procurement. We have OEM's who dictate Automatic Transmission Fluid (ATF) as factory fill in a hydraulic system. This is perhaps not the perfect fluid for the system (when looking only at the hydraulic components), but it will live and the OEM and more importantly their customers are happy. That fluid is specified by the OEM, and must be used or there may be warranty implication. We have OEM's who dictate Universal Tractor Fluid (UTF) as factory fill and required use for warranty. The same thing...perhaps not the best for the fluid system components, but their customers are happy.
We have OEM's use gear oils, HEES (synthetic esters), HETG's (tri-glycerides), HEPG, HEPR, Water-glycols, and yes, even engine oil. Each of these fluids can be used, but each of them carry their own risks and tradeoffs that accompany the reason an OEM might select them. Hydraulic systems don't have combustion or their by-products, so some of the additives in engine oil are useless. Engines normally don't have the yellow metals that hydraulic systems have, so in some occasions, additives in engine oils may harm the yellow metals in a hydraulic component.
The OEM's that these hydraulic transaxles are marketed to have specified engine oil pretty much across the board, as that is what their market and customers prefer. Their end users know and recognize the designation of a multi-grade engine oil, and feel comfortable running to the local "Walmart" and picking up a bottle for a fluid change. It works, and everybody is happy.
I'm honestly quite surprised more of these lawn tractor OEM's have not switched to environmentally friendly hydraulic fluids that would not kill the grass if it leaked. I know many golf courses run such fluids.
Fluid selection is a trade off of many things. Component life, temperature range, environmentally friendly, compatible with other fluids, one fluid per machine, fire resistant, non toxic, viscosity ranges, profit in aftermarket sales, even color can play into the selection of fluid by an OEM. What you "think" is the correct fluid for the correct application may in fact be a compromise of criteria dictated by the OEM. How you use that machine in particular might dictate a different fluid entirely.
I hope you have an open mind when reading this and realize every machine you buy isn't necessarily filled with the holy grail of fluid choices directly from the OEM. I hope this doesn't further increase your level of being "disturbed".
If you are bored, we publish some of the details around using these fluids in a manual you can find online for free (link below). While I am not a chemist, I did teach our class that we offer on hydraulic fluid selection (OEM customers and distributors only) for several years, so I do know a little bit about hydraulic fluid selection.
edited to add "link below"
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