These cabins are often built on the cheap, with minimal or no insulation, inexpensive single pane windows, small and basic kitchens and bathrooms and compact footprints. Sometimes they are true log cabins built of solid logs.
I think it's human nature to want to improve where we live. Perhaps our income has grown since we first purchased the cabin, we want more space to entertain extended family, or we're now able to work remotely. Over time our simple rustic cabin has become too basic and too small.
However, it's important to know the code issues that come into play when upgrading from an unheated (or wood stove-heated) home to one that uses propane, natural gas or electricity for heat. The Wisconsin Uniform Dwelling Code states that all one- and two-family dwellings that use any amount of non-renewable energy for heat generation must comply with Chapter 322, the Energy Conservation code. Dwellings using renewable energy such as wood or solar-generated power are exempt from the requirements (although I'd argue that it's still a good idea to insulate floors, walls and ceilings for greater comfort).
Some types of dwellings, such as those built with stud walls, having basements or insulated crawlspaces below the house and generous attic space above the ceiling, are relatively easy to insulate. Solid log homes, homes with a slab on grade floors or with vaulted ceilings having no attic space above, are more difficult. If you're considering purchasing a rustic cabin, it's worthwhile thinking about whether you may want to make this a full-time home in the future and about how easy it might be to bring the cabin up to code if you do.