As the author, I should probably tell you my intentions here.
You can't have the module active without Cities active (something that bugs me, but isn't too easy to get around). I honestly do not see any point in having 6/7 cities which all have stables. Granted, that's my opinion, but it was absolutely definitely intentional to block it for Polareia Borealis as it's a holiday themed "reward" town that is supposed to be as different as possible from the other towns - if you delve into the themed shops written for it, they are all quite generous. They have buffs that are easy to get and cheap, they have very few negative things and the odds are all in the favour of the player.
For that reason, the place wasn't designed to be always on.
As the core is by default, the stables are already in two of the towns. If you want them in all towns, a module isn't difficult. I think for something that isn't likely to be in the best interests of all admins it shouldn't be altered in the core.
I also coded the Ice Caravan (the method of getting there) so that travel is unlimited that day and doesn't cost any extra to go back and forth. If you artificially put a stable there, players can pay 5 gold then get to a stable from anywhere without using a travel / forest fight

The other repercussion is some of the mount modules. One sets where animals appear. You'd need to take that into consideration because Polareia Borealis isn't in the dropdown menu when you select where each mount appears.