They're not happy and want to cause drama.
Why expose anything that has nothing to do with you?
People spend too much time in other people's business.
Maybe they are envyous,like fibo777 states.
Now if the person is telling on themselves,like calling or emailing the spouse/girlfriend/boyfriend, telling all that the person is doing for them or with them,they're looking to cause drama and in the end that person will continue to cheat just not with them.