Seems like the worst case scenario is when you mix solar panels with load on the same circuit.
800W at 110v is about 7 amps. Suppose you have a 7 amp solar inverter plugged in as the first thing on that circuit. Further down the line, you have 22 amps worth of load. The circuit breaker sees 15 amps, but the wiring between the panel and the load is carrying 22 amps, which would ordinarily trip the breaker but it doesn't because the solar setup is masking it.
I'm not an electrician. Perhaps wiring code has enough safety margin that being 800 watts over on some circuit isn't actually a serious safety concern, but I could understand why this might not be allowed.
The EU is on 230v, so maybe it's a bit more practical over there because you can add twice the solar capacity for the same amount of additional current you'd have in the US, so they aren't eating into the safety margin quite as much. Though it seems they also use thinner wire gauge generally, so maybe the difference is moot.