The way power is shunted into the grid is very simple.The inverter you have senses the line voltage, and by keeping its voltage a tiny bit higher, "knows" that the power will flow out. nothing to sense and push against and it will idle likely doing an on and off test routine till if finds something to push against. Vs an off grid inverter that is stand alone, and self referential. There are hybrid off/on grid inverters, but they will quickly stop sending current, without a reference voltage, and drop back to battery/solar. As to legalities....,if there is no regulation adressing it, its legal, beurocracys hate that, but there it is. Its not like they wouldn't pin an (unlikely) accident on you either way, so..... and with no batteries, there is more risk in dropping the panel or another mechanical failure than anything electrical