USB-C actually has pins dedicated to application/debug use (SBU1 and SBU2). "SBU1 and SBU2: these are low-speed lines used only for Alternate Mode and accessory mode. For example, with DisplayPort, AUX+ and AUX– transmit over the SBU lines. For audio adapter accessory mode, these lines are used for the microphone input and analog GND."

Cost. It's a clever hardware hack that saves money, and 1 cent saved over 100 million units is $1,000,000!

Mining rigs were using USB-C to transport PCIe x1 by the truckload. Because USB 3.0 cables are dirt cheap and are capable of transporting signals at PCIe x1 data rates.

https://www.aliexpress.us/item/3256809341543494.html

4G/5G Telco equipment (ab)use HDMI connectors/cables in a similar way.

As long as it's not intended for user to plug in, it's fine. Many other vendors also do that.