Danger/Risk of Plugging USB Devices into Powered USB Ports
Your logic is accurate; there is no inherent reason to expect harm. The device will just see a +5V source. Various power adapters have the data lines open, shorted, or connected through a resistor network to the power lines, depending on which of the standard or non-standard “this is a charger” schemes they use; but I believe the USB specification requires devices' data transceivers to be robust against this.
Possible hazards:
Some power adapters might be poorly regulated, or have higher voltage for faster charging of the device they are meant for. At a minimum, check the label on the power adapter to make sure it is specified to output 5V. (Higher current rating, i.e. current capacity, is not a problem unless it is a “linear” power supply, which is unlikely in any modern portable gadget.)
An especially poorly-designed peripheral might be damaged, not by the power adapter directly, but by being connected to a power source for a long time without being communicated with (staying in initial power-on state). This is unlikely.
But, what happens when you plug a device into a USB power-adapter like (B)? I’ve plugged MP3 players (and of course the GPS device it came with) into it and they simply charged. But what would happen if I plugged a mouse or flash-drive into it?
Nothing. The USB device will power up, and attempt to communicate over the D+/- lines. Without any communication, the USB device will either go into standby, or keep polling the data bus (depending on how the hardware is implemented).
Logic says that they would not be harmed and act exactly like they would if plugged into a powered-hub, but is that indeed the case?
Yes. You can't force more current into something, the device itself draws as much current as it needs to drive the load at the supply voltage. Since the device "sees" a normal USB hub, there is no condition that would cause the device to be damaged.
I believe you're right.
In particular, the wall adapter should be just the +5v and GND lines wired up like you mentioned, so a mouse or flash drive should just get power and nothing else. It'd be like they're connected to a computer in standby.
A USB hub would also have the two data lines connected, but it's just to the hub circuitry which is meant to talk to devices anyway.
I can't see any of this causing problems. If it does, the adapter isn't conforming to the USB spec and should not have a USB port to begin with.