It is not "back to back" ticketing
A back to back routing would be (ex.) LAX - DEN (DEN - MSP - DEN) DEN - LAX where you are "actually" flying LAX - MSP; but, ticketing as such would offer a price benefit. Or, an extension to the tail of a reservation which would offer a price benefit.
I can imagine the argument, sir this is a back to back routing because you are going from LAX to LAX?!?!?
Next time just go to nwa.com and use the multi segment tool to purchase a LAX - DEN - LAX - DEN - LAX