brining and flavor infusion seem fine, provided the marinade isn't full of sugars or seed oils. Any marinade or sauce bought off a store shelf should be inspected carefully for unwanted sugars and oils, as well as other preservatives.

All of that depends on what your individual purpose is with respect to carnivore - if you are wanting to stay in ketosis for example, then the sugars aspect is more important. If you are wanting to eat based on some sense of raw nature or history, perhaps you wouldn't want to include certain fruits or plants or other products that would not have been used. If you are simply out to change your body chemistry and gastrointestinal processes, then avoiding even a hint of sugar (to avoid as much insulin production as possible) and any plant-based oils because of their effects on digestion and the ability of the body to absorb nutrients becomes more important, and really watching salt intake may also factor in.

Then there's the social aspect - while I might be fine eating a tough steak with no seasoning at all, just topped with some unsalted butter to add more fat, my family probably would not be, so cooking for others means relaxing my personal preference (or convenience).