Being tied up is really no difference from being fenced in.
I beg to differ. A dog that's tied up can become tangled in the leash/chain/rope, twisted around the post, tree anchor, bushes etc. A dog that's tied up, unless it's tied up within a fenced area, is vulnerable to passers by, be they animal or human. A dog in a securely fenced environment has a greater degree of protection from both and much less of a chance of hanging himself.