A fuse protects against overcurrent. It does nothing against overvoltage, or worse, reverse polarity (which was what fried Benson Leung's device). USB-C has a maximum of 20V, and that's only when negotiated by both devices; many devices will only expect 5V. If an untrusted low-quality power supply fails, it can put the full line voltage (alternating from -110V to 110V or more) into the USB cable. Protecting against that would add a lot of bulk and cost to a device. AC powered devices, on the other hand, already have to deal with the full line voltage (which includes negative voltages) all the time, so there's no extra cost for them.