Artists should write songs and albums about the things they know and that inspire them to create. If an artist is going through a bad breakup, it's natural that it would bleed over into their music. There's also the sense a lot of times that it's exploiting a public event to sell units (like everybody knew about Britney and Timberlake, and both of them took shots in a not-so-veiled way), but whatever.

Legally, I don't see any issue with it either unless it tilts over into slander. There are laws covering that.