Detergents don't wash away ZDDP and anyone who says otherwise either doesn't know what they're talking about or is doing a really poor job of trying to explain a chemical process in laymen's terms. Some detergents (and they may not be the ones used in diesel oils like Rotella) can decrease the effectiveness of ZDDP. For example, there's been a study (see link at end of this post) that has shown "conclusively that in the presence of [...] calcium sulfonate [(a detergent)] under sliding conditions, ZDDPs do not form long-chain polyphosphates that have been associated with antiwear action." So detergents can make a difference, but who's to say what detergents are being used in what oils (mostly proprietary information).
For the record, I agree with what you're saying Junky, I'm just trying to explain where people might get this whole notion that detergents "wash" away ZDDP.

Article abstract: http://www.tandfonline.com/doi/abs/10.1080/10402000308982660