[Quoth]

The original USB 2.0 host spec only allowed 2.5W per port (500mA). Apple first had a non-Charged USB peripheral that needed more than 500mA (1A = 5W). For a long time some USB devices that were not charged came with Y cables to use two host ports for power. Such as 4G modems in 2008 and still even recently some USB HDDs.

The 30W is SIX AMPS at 5V, no single regular USB device uses that, 10W is the highest and most of those will charge at 2.5W if the host port doesn't have the signalling they desire.

USB-C is a completely different thing as the Gadget doesn't just "see" what current is available but can "ask" for more than 5V. IMO should have used a different system altogether. It's a bad design.

There is no absolute universal standard for more than 500mA (2.5W) on regular USB - A style host ports. Apple invented their own and some use it. Others use a different setup.