Post sex STD tests are a strategy--but what do they cause a person to do? Give up sex? Have less sex? Have more sex in order to "get even"?
Testing before sex is a way to keep some sex from happening, and, consequently, prevent an STD. Post sex STD tests don't do that. Pre sex STD tests will save some lives--some people will back out of the possibility of sex with that partner. Others may be way more cautious. Still others might wait for non-HIV STDs to be cured, if available.