Yeah, they used to do that. They stopped because it burns passenger time -- can add 5 to 10 minutes to runtime. It was decided that it was better to pull in forward, unload, then back out and wye without passengers before loading the next group of passengers. This is correct, since all trains terminate at Chicago.
And at Denver the fuelling point is at the far end from the bumper blocks, and the platform is short enough that if they didn't back in, some of the cars would be off the main platform onto the service platform. So they have to back in. Plus they have to either reverse on the way in or on the way out, so it goes into passenger time either way; whether to do it on the Denver-Salt Lake segment or the Denver-Chicago segment is six of one and half a dozen of the other.