This behavior comes down to a fundamental design change between USB-A and USB-C.
All previous flavors of USB-A (which by the definition of the earliest USB specs, going all the way back to 1996) are Hot receptacles, meaning that if the source or USB host is turned on, even if no device is attached, one of the pins (called Vbus) in the USB-A receptacle has 5V live all of the time.
This “Vbus Hot” behavior was just the way USB behaved for 18 years. USB-A devices (like thumbdrives) were given this basic assumption that if they were plugged into something, they’ll just magically get 5V immediately from whatever source or computer they were plugged into.
For a lot of reasons, this was not an optimal design. Having a 5V hot receptacle meant that if a piece of metal fell into the USB-A receptacle, a short could form, damaging the source.
USB-C f or Sources, is designed to be a “cold” receptacle. The main power lines (Vbus), are guaranteed to be 0V until a sink device is attached by way of a different configuration channel pin. If and only if the sink correctly signals to the source that it’s a sink does 5V power flow like it used to in the USB-A days.
All correct USB-C sink devices need to do is present a simple pull-down resistor (called “Rd”) on each of the CC pins on their side (either receptacle or plug). It’s clearly documented in the USB-C specification that this is the way things must be done.
However, many devices that switched to USB-C didn’t follow these simple rules, and the companies didn’t test their devices with anything except the A-to-C cable that they shipped in the box…
Long story short: USB-C change the way that power flows for safety, but many companies didn’t implement the spec correctly, going based on incorrect assumptions of how things used to work in the USB-A/USB-B era.