Sam - it's in the member's book. And no, you don't weigh pineapple etc with the skin on. the whole point about banana's is that you weight with the skin on to be better able to guess how many points a typical banana would be if you don't have scales available.
WW don't always like to you weigh your food anyway. It is a very useful tool but when eating out etc. you should be able to guess - a portion of cheese at 4 points is the size of a matchbox, a 2point jacket potato is the size of a lightbulb etc. They recommend weighing at the beginning and then if you plateau or gain weight. Otherwise you should be attempting to find ways to guess how many points are in typical portions of food to help you cope with maintaining when you reach goal!