She was probably under 18 at the time. (I don't know, I'm just guessing, but it seems likely.) Furthermore, if I'm the editor of the Tribune, I don't want my reporters and/or columnists getting involved with people they are writing about. It hurts the credibility of the entire paper, even if the writer discloses the relationship at the time. That alone would justify Greene's demise in my mind. Especially since Greene has a history of writing about and therefore dealing with troubled (and emotionally vulnerable) youth. It doesn't say that Greene wrote about her; only that he met her through the column. That could mean any number of things. And we don't know her exact age -- she could be 18 or 19. Unless there are some more details forthcoming, it doesn't seem that Greene's sins deserved a forced resignation.
Most media companies, papers, radio, television, etc, have a moral clause in their employment contract.
You only have to be accused not convicted.
Iam sure this is what the Tribune used to get Greene's resignation.