TrueHD and dts-MA each use different methods of producing 5.1 outputs from 7.1 encodes. I suspect neither is technically a downmix. But, neither of them discards the rear channels. The 5.1 output includes the material in the 7.1 output, properly mixed for a 5.1 presentation. Since there are almost no 7.1 theatrical mixes, most of these soundtracks actually begin as 5.1 and get remixed for 7.1.
That's interesting about Toy Story 3. TS3 is the first movie mixed using Dolby Surround 7.1. Perhaps it was easier for the studio to simply encode the 7.1 theatrical mix as one track and the 5.1 version that played in most theaters as another, rather than integrating both mixes into a single dts-MA package. But, that's just speculation on my part.