It's not quite as simple as that, when you're in the car and going balls out everywhere (I know F1 isn't like that because of other stupid rules, but hear me out :P) you literally don't have any grip left available to change direction / avoid an incident by carving a tighter turn, so you HAVE to run wide. This is fair enough IMO.
Running wide every f**king lap (Vettel) is definitely not on though, and it's relatively easy to police. There are pressure pads at a lot of UK circuits now which are linked to cameras - you get a pretty picture when you roll over them. Put the pressure pad a few inches further from the white line than the width of a car and hey presto, instant evidence that you WERE off track, then apply the penalty.
There are some cases where running wide does lose time, but with all these new concrete / astro run-offs and stuff I think these are few and far between. They manage ok at Monaco, and a 5s penalty is a lot better than having to rebuild a corner of the car...