It doesn't seem fair that a casino can take all of your money with no issues, but if you take too much of theirs they can ban you.
Casinos don't ban winners, they ban high rollers who leave the table as soon as they are up as they are playing to a strategy that's deemed abusive to the promo system.
They usually give high rollers slightly different rules which results in a slightly higher winning percentage for them compared to the plebs (casino still has the edge) They also offer comps on stuff and sometimes offer cashback on losses, say 10%. This long term results in the player having the edge across all casinos as long as they can take advantage of these promos, particularly the cashback on losses.
However each casino house still has the edge over the player, but walking away as soon as the player is up identifies the player as someone trying to game the comps, promos and cashback system rather than being a genuine high roller. End result is a ban.