Shad,
In order to be fully backwards compatible with regular DTS, all DTE-ES discrete titles have the rear channel info also present in the rear left and right. This makes DTS-ES Discrete not only backwards compatible with DTS 5.1, but DTS-ES matrix as well.
As for why some manufacturers...
I have an ES-Matrixed only receiver, and every ES-Discrete title has always been auto-detected by my receiver and played back in matrixed mode.
-Lyle J.P.
Minority Report is now the second disc to be flagged correctly on the Dolby Digital for EX, but incorrectly for the DTS ES (Atlantis: Collector's Edition being the other). Quite odd considering 90% of the time it's the other way around. In fact, Jurassic Park III is the only disc on the market...