The key point you're missing is not simply allowing access, it's allowing modifications. In your property example, that's like telling someone that hey, you can go ahead and paint my house for me and then complaining when they file a lien on the property because you didn't pay them.
Or, perhaps a little closer to the linked in example, many states have laws that will allow people to claim ownership over property (land, in this example) they have maintained for a certain period of time with the knowledge and (tacit?) approval of the property owner. I forget what the legal term is, but it does exist.
Common-law marriages, if still on the books in any states, are perhaps another example.
I believe there have also been court cases where a person sues a former housemate who allowed them to live in the house without a rental agreement due to a "relationship" getting a certain percentage of ownership awarded to them because they were able to show spending time and money maintaining/improving the property.
I guess my point is be careful of anything that may be interpreted as sharing or giving up ownership rights.
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