It looks like your subpanels have two hots, a bare ground wire and no neutral.
That would be fine if all they were supplying were 240 volt loads.
The real danger here is if that ground wire corrodes or gets loose you will have both a floating neutral and potently dangerous voltages on the grounds/cases of devices.
The reason they want two wires going back to the main panel is that if one(or both) open up there is no way that you end up with a neutral connected to a floating ground/case of a device.
1. It is not necessary dangerous right now... but the problem occurs over time when connections loosen and things corrode.
FYI: I've done this for temporary/testing wiring since it does not matter in my case since it's going to get destroyed anyway. (destructive testing)
2. It's not up to code and would not pass the type of inspection you need to sell a place.