That seems like an issue with checking though. If you know people can switch bikes mid race, meaning it's allowed by rules then it is simply stupid to only "double check" the winning bikes that made it to the finish line. Obviously you would need to check every single bike someone used during the race. That's different from someone illegally changing bikes.
This is what they do, for what its worth. Every team bike is subject to random or suspicion based inspection both pre-stage and post-stage. There's also in-stage monitoring that flags riders or their equipment for additional investigation.