It must have been clear enough for every other rider in the event to abide by the rule
Nope. apparently there were several riders doing the same thing. They just made an example of Barel.
I think it's a bit of a grey area and they overreacted, the punishment didn't fit the crime. It was during practice, not the race itself and Barel would have been, in real world practical terms "stupid" to not do what he did. His accommodation was closer to the trails he was practicing than the lift.
There is a deeper subtext here though. A lot of the riders seem to still be smarting from Barel's victory at the first round where he apparently used motor bikes to shuttle during midweek practice leading up to the event. This is what I believe put Peaty, Minnaar and Lopes' nose out of joint. However, according to FB, many riders were already there before him, were already doing exactly that several days earlier so he was only evening the playing field for himself.
The Syndicate boys arrived later again, and couldn't get the same practice. This is also apparently why Minnaar tried to hound dog Barel on several stages, to follow his lines, but by his own admission, Fabien was just much too fit and strong and he simply couldn't keep up. Shuttles or not.