It's far from arbitrary. It is expressly designed to stop people from searching for cheap(er) routings and then changing those cheap tickets to what they wanted in the first place. All DL is doing here is selling you the new ticket for what it would sell the same ticket to anybody else.

As to not cancelling the remainder of the ticket if you noshow for one segment, that won't work. Period. The system auto cancels. That too is pretty universal. Only place where you can make that not happen is on tickets purchased in Germany, but those tickets are eye-poppingly expensive.