Hello, Sadly the PS Plus subscription is not something decided on by Norsfell but rather by Playstation itself. Not sure how other games go about NOT having to use it, but for Tribes it is a need to have thing according to Playstation. Also, although offline gaming does not sound like a difficult thing to do, from a dev stand point it would take a great amount of time and effort to get it put into the game as it would not use servers, but your own playstation console (or PC, xbox, etc.) and the coding for that is completely different and would need to be almost completely redone.
Maybe in the future they will be able to get this into the game, but currently they are working on seasons and other things that have been requested by players as well.