If you see papaya you can take it, means that if others let down their guard, for example by saying something stupid then you have to make fun of them according to the papaya rule. Back home we would say that the friends who laugh at each other are not very good friends, but here that is not true. Here they would still be their best friend, because it is not you who should be blamed when they say something stupid that they should not have said.
The expression is not meant to scare you, but merely to think smart and take care of your belongings and what you say and do! After having actually given papaya in Colombia I will remember this rule in order not to have my thing stolen the next few months while travelling through the rest of South America.
This can happen all over the place, all over the world, Colombia may have come up with an expression for it but it is prevalent throughout Latin America, and you can see it in the West as well. It is a global phenomenon. People very often view kindness as weakness, and they are repulsed by gift giving, and view themselves as having taken what you have freely given of your own volition. It is a way of thinking which makes the world worse off. It’s pathologically narcissistic way of viewing human relationships as purely transactional.