My best bet is that people pick the herbs without planting a new one. Being a responsible picker is not just a matter of avoiding overpicking (standards for this is lowered all the time though, eh, Lilandrin!)
Every time you pick a herb you will need to pick an extra and plant it, -you cant simply know if you have just picked the 'mother-herb' in that location, and this method will ensure there is a new 'mother-herb' to produce new herbies.
(There could be other reasons though, like, what is the need for natural growing when some people still have shops where certain herbs has gone crazy?)
Best regards, Black Blaatand
Written by my hand on the 27th of Agamnion, in the year 958.