Can somebody explain to me what Lukaku actually did wrong?

So this has been bouncing around my head a few days. Inter fans are very angry at Lukaku at the moment for having discussions with Juventus about a potential move. And to a degree, I understand that. He is a player they supported in tough times talking to a rival.

What I don't get is how Pirlo played for Milan, Inter AND Juventus, and is a much beloved player. In fact there is a whole list of players who have done this, Zlatan, Baggio, Cannavaro and Vieri to just name a few noticeable ones. I always hear about how transfers between rivals are just a bit different and more common in Italy than the Premier League, so what gives here? Why is Lukaku a snake but those other players are Italian legends?