Route maps are most likely indicating that NW codeshares on this route - the caveat is that you need to have a NW-metal flight somewhere on the itinerary to be able to book the codeshare. For example, you could price out MSP-SLC-LAX on NW, with MSP-SLC on NW metal and SLC-LAX on DL metal, but just doing SLC-LAX wouldn't work.

Booking through DL is your best bet for this one. WN also does SLC-LAS non-stop.