I'll be very brief, but perhaps provide some clarity.
As I have been in both guilds which have Ult herbs and have had personal experience in the matter, I know at least as much as most people.
Ranger's CAN pick wurtfoil.
But naturally occuring wurtfoil is so rare that the skill is practically useless. Try Presciencing it sometime.
All Wurtfoil in the land dies off every new year, and perhaps a few paltry plants grow in every new year as well.
The only practical source of Wurtfoil is an animist evoking it.
So other than 1 or 2 or 3 plants, it is impossible to comeby without an animist.
So you are correct in saying that no law governs its picking, other than that of the Gods who have stipulated it is not to be sold.
But the true way of things is that it is so rare and impossible to comeby naturally, that again, the ranger skill is essentially pointless.
Written by my hand on the 9th of Springflower, in the year 1042.