It’s not impossible at all. People have done this literally for decades. Classic WoW only exists because people hosted their own seevers and Blizzard wanted in on the money. Star Wars Galaxies the same. I think Everquest 1 as well. And probably others as well.
So why does this law need to exist if everyone is doing it and has been doing it for decades?
Because they can be sued for that. Have been sued for that. And while it is possible to reverse engineer this stuff it is incredibly hard to do. So games with smaller fanbases might lack the manpower to achieve it. Or the game was made in such a way as to make reverse engineering impossible.