maleficium

Power can stem from raw might, land, wealth, prestige, or magic and local princes would like to control all of these things, especially magic. But doing so is easier said than done.  

Maleficium

  Most realms have some form of "the Maleficium Law." In short, this custom means that's it's not a crime to cast magic but if magic is used in order to harm someone or steal something the criminal actually committed two crimes. First the regular crime (theft, murder, rape, assault, etc) plus the charge of maleficium which is a catch all for any baneful magic. In other words, the use of hostile magic is an aggravating factor. Law enforcement will hunt down the perpetrator more vigorously and the punishment will be that much more severe.   Most princes give leeway to the priesthoods (or at least some of the priesthoods). If a theurgist or a favored soul commits a crime, the local ruler will often turn the offender over to their own religious leaders and they have to deal with an ecclesiastical court. This is rarely a mercy, because most priesthoods are embarrassed by the misdeeds of their followers and are very harsh on anyone that causes them to lose face.   This is different when the spell-caster caught committed a magically aided crime is part of a religious faction that is not officially recognized by the law of the land, in which case the malefactor will be charged in a secular court. A lot of magical crimes committed by Testers or Children are not only not considered crimes by their respective priesthoods, but they are actively encouraged.   Arcane casters do not normally have any special protections or separate courts at all though if the accused is a member of a Feudal Coven, they might be able to get tried by their coven rather than normal authorities.  

Routine Regulation

  It's rare to have blanket regulations and laws covering all divine magic. Princes may try to restrict worship for deities they find problematic in which case they will make decrees regarding those deities' followers and not about divine magic specifically.   Rulers are more apt to be strict on arcane casters and make blanket decrees. Most rulers that declare all arcane magic off-limits save for their chosen retainers find this only encourages rebellion rather than suppresses it. Most mages view a prohibition against casting all magic equivalent to a decree essentially saying "you are not to allowed to exist."   Most rulers compromise by issuing licenses to cast magic in their realm. These licenses to practice magic are usually pretty cheap, typically ranging from 5 gold pieces to 100 gold pieces per year. The point is not to generate revenue. The point is for the rulers to know how many mages are in their realm, what their names are, and ideally what they can do.   Sometimes princes will try to restrict who is or isn't eligible to be trained in wizardry but like blanket restrictions on all magic, this can backfire. Wizards are a prideful lot and don't like being told who they can or not teach the mystic arts to. If anything, restricting who can learn wizardry is often not necessary because it's expensive and time consuming to learn wizardry and most wizards prefer to apprentice mainly upper class individuals because their parents can pay them well to do this.  The people who have that kind of money and time are more often than not on good terms with the local lord whether they are high born or low born.    

Regulating Reagents

  Since reagent trade involves actual physical goods changing hands, this is easier for princes to regulate than magic spells. Just like certain forests only allow the local lord to hunt there and certain bodies of water only allow the local lord to fish there, it's very common for the local lord to bar anyone from gathering reagents on the lord's land without his permission.   Some lords want to regulate reagents trade to keep tabs on all the local spell-casters but more often than not, they just want to make sure they can fatten their purses with a reagent duty.