If the inner components of the taillight is compromised (e.g. melting), they will & have to replace it. Happened to me, have a 328i LCI as well. The reason they do is because of liability issues relating to future electrical problems (e.g LED housing). This is only happened to one of my taillights but it was cheaper to get the set because usually if one goes, the other will follow. Hope you get it fixed, its not that bad on your wallet
