It's not that it's too much work, it's that they're trying to protect revenue (they keep more money when they fly you, instead of you flying someone else). They used to weight the display results equally among all flights, but now they bias it towards their metal. Third-party sites like Expedia don't have this type of bias because they don't care who you fly.

While inconvenient, I tend to agree with this logic. A codeshare partner is still a competitor, and an airline should drive business towards its own metal. I can't think of any other real world comparison to the codesharing practice, though. It'd be like if McDonald's had the ability to sell you a Whopper. I doubt they'd advertise it, or put it at the top of the menu, but if you asked for one, they'd give it to you.

Btw, other carriers do the same (e.g., try getting a NW-coded, CO-metal flight out of the NW site or a NW agent, it's equally, if not more painful).