Huh, the wiring just supports power spontaneously coming from an exit point rather than an entry? Is that commonplace?
Why wouldn't it? Electrical wiring isn't a one way road, electricity (this is an extreme oversimplification, especially when it comes to AC) will always flow from points of high voltage to points of lower voltage. That's how solar inverters feed into the grid. Raise their voltage a tad bit higher than the grid and match the frequency and phase of the grid until the outflow matches their maximum available power.
Is that commonplace?
That is a hard question, because this isn't a feature, it's how things are. Only thing one needs to take care of is that the solar inverter doesn't deliver so much power that the circuit can consume beyond the circuit breakers capacity, otherwise the breaker would be rendered useless. That's why these small plug inverters are limited to 800W in Germany, that puts the entire possible load on a 16A circuit into the general upper limit that is still within the safety margin fro 16A circuits.
EDIT: Now before someone gets the bright idea to connect their diesel generator to the grid this way: Don't. It will not be in sync or phase and that will make something spectacular happen, but it will not supply the grid. Either have an expensive generator that is able to sync to the grid or have a grid disconnect and switchover in front of your generator plugin socket.
EDITEDIT: Also please never connect an island capable solar inverter to a plug. The ones described above are safe that way, because they wait for grid voltage to be available before they do anything, so there will never be high voltage on the open plug. An island capable solar inverter does by definition not do that. There will be high voltage on the plug and it will kill you and it will hurt like a fucker the entire time you're dying.
On some level, water would work the same way. If you were to collect water from somewhere, feed it into a pump and hook that up to your kitchen faucet, as soon as you increased pressure a little above that of the public water pipe, water would flow backwards from your faucet through the pipes in the house into the public water supply and your water meter might run backwards, depending on its construction.
disclaimer: Unlike freshly harvested AC electricity from a solar inverter, home collected water does not meet the hygiene standards for public supply. Absolutely do not do that either.