I agree but only the I version, otherwise siege bases could brew potions without limit.

Allowing the Siege Associated Potions to be brewed in peaceful towns would diminish importance of the unpeacefull towns in both it's value and it's distribution within a nation, specially when such are "closely surrounded" by peaceful towns.
Taking Britain as example, although mostly concise in the region of the British Isles, there are some other strategically placed towns such as Barony Of Kloshire located in Greenland (possibly positioned to surveillant actions of nations such as Nordland and Iceland), Svalbard present in Arctic Region (likely to benefit Cold's Biome Status for Realistic Plant Growth) and Omega in Azores (as a movecraft station for Atlantic Region control).
In the case of Barony Of Kloshire and Omega, the ability to produce such potions without the risks associated with unpeaceful towns grants them higher capacity to perform Naval and Land Sieges instead of requiring supply from their closest unpeaceful town, Dunlace Castle.
Nevertheless, this type of strategy could deem potentially dangerous of the nations as it requires only the successful invasion of Dunlace Castle to render full occupation of Britain.
Another aspect that makes me believe that this change may not be beneficial is the provoking of greater amounts of confrontations between nations that despite being considered an essential point of CCNet Nations, can be done to the extreme, bringing more destruction and toxicity than good sportsmanship.

i can answer your question: "Why is it not possible to brew potions in peaceful towns?" on which your argument is: "Residents of towns get terrorised all the time it is only fair they can make pots without having to travel all the way to their fort"
nobody says u need to fight in peaceful claims, besides, u can trap them in your claims.
and about selery, selling potions doesn't make any sense, how are u gonna deliver it? and why would someone spend money for some time and few cheap materials, besides heal potions, people already spend enough on pots and i don't see any situation where i would want 200 dubs of potions to buy. and if peaceful towns can brew potions, in sieges, attackers will have unlimited potions even if u won't allow it in ,,siege bases". pots are precious for a reason. u dont need defence in peaceful towns