Because Fluther topics only work when everybody uses the same terms. For example, imagine I was a newbie interested in discussions on kids. I would, on joining Fluther, follow the topic “children”. It would not occur to me to also follow, “brats”, “offspring”, “tykes” and “younglings”, so questions tagged with these terms would never reach me.
It makes sense to use the most simple terms you can think of, as well as the most widely known. In your case, Fluther should have suggested, “journalism” upon your entering the first few letters of the word. Instead of “Good Samaritan”, a more broad term such as “ethics” would alert the people who might be interested in the thread.
When you use topics Fluther does not know, that means you are creating new ones. In most cases, it makes more sense to look for a synonym the site – and, consequently, the fellow jelly interested in your thread – already knows.