I think it's important to note that the second blood draw is to detect drugs which are currently banned, but not known how to test for.
Err, yes. In the case of chess, all computer assistance is always banned, but some algorithms may not match current patterns of behavior.
In the case of blood tests, there is a cat and mouse game of changing how the drugs are administered or other chemicals in the blood that reduce the effectiveness of the tests. Later on the presence of those other substances may be used as motivation for re-testing a group of people.
Re-running the games (especially to check for new techniques that aren't public at the time of the event) would be in the same spirit.