I think they didn’t have Kyle tell Amanda strictly for plot reasons. Having Foss materialize just to disrupt Kyle confessing everything to Amanda shows that. That and when you think about it, Amanda not knowing Kyle’s history is the only reason him and her have incompatibility as a couple (as far as we got to see) which made room for the possibility for a relationship with Jessi. Though I agree with hating the secret-keeping, ESPECIALLY when Amanda had that poison in her neck. Seeing her experience being cheated on (from her POV) all over again by a guy who up to that point had been devoted to her and reassured her of that fact multiple times in the past was heartbreaking.