Of course there are always exceptions but the general pattern of cultural development is that you are ready to face the possibility of alien life when the chance to get out there becomes a material reality. We are talking about a guideline for Starfleet captains when they first become aware of the existence of a new species and have no data on them at all. Suppose Starfleet does not detect antimatter, decides to conduct duck-blind missions and then realize that these guys are ready to welcome alien visitors. Then you obviously initiate first contact. In the case of first contact you have to err on the side of caution.