People will swim in the ocean, even though there are corpses in it somewhere. People will not swim in a pool with a corpse in it. Humans all have a corpse:water ratio that is acceptable for them to swim in.
If people stranded on deserted islands don’t clean up their ‘HELP’ signs after help finally arrives, rescuers would spend hours looking for someone who isn’t there
Signing contracts with blood actually makes sense. A written signature can be forged or ambiguous, but a DNA test will always show whose signature it is.