Why poaching is bad?

simple it is illegal. If you want an answer which is less simple-minded, game laws are put into effect to control and protect wildlife populations. In the past, unrestricted hunting has literally driven a good many species to extinction. A number of iconic animals like rhinosceroses and tigers face extinction and the laws have been put in place to prevent that from happening. So basically if you want your children to see a rhinosceros which is actually alive at some time in the future, don't kill one now.