Tracon, you're suffering from the "HIP check". If the fare you are interested in is an MPM-based fare, that is, one based on a maximum permitted mileage, stopovers are calculated based on the most expensive routing. E.g., if ICN-YVR is more expensive than BKK-ICN-YVR, then the fare will be priced as ICN-YVR.

The OZ fare is an example of this.

However, other fares, such as the NW fare, are routing-based. As long as you follow the routing allowed in the fare rules, stopovers are free, unless the rules state otherwise (some rules say "no stopovers" while others say "stopovers allowed a $x/stopover").