I don't think it "must" be? Tomatoes are very common in that situation because their seeds can survive passage through the human digestive system, but if a different plant's seed happened to coincidentally land there there's no reason it couldn't grow too?

Would you be able to eat fruit from that? Does bacteria travel to the fruit from within the plant?

Well, animal feces is commonly used as a fertilizer. I think it would be bad news for a plant if bacteria were traveling around inside it.

People reusing human wastes generally try to compost it for a while, I think, probably partly to reduce the risk of diseases, but probably also in a large part to remove the foul smell. Shoveling shit isn't fun.