But should there not be a diode upstream of each USB ports to prevent this?
The poly fuse is not mono directional is it, so that no diode is required?
If the poly fuse were operational it should have blown in a few seconds and shut the RasPi down. You are running HDMI and have everything else running off of the hub, it appears. You are not using ethernet so that is lessening the draw somewhat.
Another question is will it boot this way, or does it just continue running after it is already booted? The draw would be lower as you are not running power through the linear regulator which is a power waste. So less amperage to run it.
So does it power this way with both ports or just that lower port?
For this not to fry your Raspi, and also be able to work this way, there would have to be several serendipitous events.
1. A solder bridge, or a faulty poly fuse.
2. A failure of, or no diode upstream of that port. (or another solder bridge)
3. Two power Supply units that are so close in voltage that the voltage difference is tolerated; the RasPi PSU and the hub's PSU.
Believe me mine will not do this. It will power the hub and the power light will stay on the hub PSU until I unplug the Raspi PSU. Since I only have the one I really cannot test this to the possible destruction. If I only had a second RasPi to throw upon the Altar. It would be interesting to see what dom or Gert would have to say about this.
MarkSpireWeb: I would be very careful with that RasPi. It does not have the normal protections that other RasPii have. If you switch out one of those PSU's you may just see the Magic Blue Smoke. I am surprised you already have not seen it. But you have a unique little berry there among the many raspberries that have been produced. It shows unintended consiquence and possibilty.