Isn't the point of a cannon rush to build the first cannons where they can't be seen?

Surprisingly not. The trick is usually to build pylons (or other cannons) such that they protect the cannons from being attacked by probes. Building them out of sight is usually too slow as a rush.

Still, he didn't do that either.

In the beginning of SCII I only saw people trying to hide it. But I guess the strategy evolved, interesting.

Sometimes you may see a photon cannon used to deny an enemy's natural expansion to try to gain an economic advantage. Depending on the map and matchup, it may also complicate the enemy's early attempts at scouting and aggression.

Typically, you don't see more than 1-2 photo cannons, because you don't usually want to "over-invest" and lose what advantage you gain.

This is not how a player would do a cannon rush, it needs to be hidden / at the edge of the opponent view.

That's inaccurate. The best cannon rushers generally build them visibly, but not just anywhere. If you look at someone like QuasarPrintf as an example, a player that keeps a fairly high rank on an account that literally only cannon rushes (there is no anonymity, no pretense about what's going to happen), he wins despite people knowing what's going to happen and putting the cannons mostly well in view of opponents on a lot of maps.

Printf is part of a fairly small group of cannon rushers that don't simply see it as just another cheese, because what generally defines a cheese strat is that it can be easily countered if you know it's coming; not so with their cannon rushes.

Now, with that said, Printf (or any other "I always cannon rush" player aren't winning tournaments), but that's partly because not many players decide that they want to stake their development on any one strat like that, and if they do, it'll likely be one that's deemed more legitimate by the community.