Designing systems around what people should do, as opposed to what they actually do, has proven time and again not to work particularly well in practice. I'm sure you've seen countless examples of how people track paths through manicured grass fields. The landscaper will complain about how people should walk and they'll put up signs to no avail.

The fact is, we (including me, BTW) are frequently wrong about a lot of things, and when there's little riding on it, we can ignore that most of the time. With subjects like medicine and law, however, where a mistake can cost you your life or lots of money, we want to make sure people are getting the best advice possible. That's why we require licenses to practice medicine and law, and we have governing and ethics bodies to regulate how professionals operate their practices.