Probably someone will have a better answer, but I'll give my opinion: The founder's grandchildren are the last generation most likely to have known the founder in real life, and to have experienced the beginnings of the settlement, which would qualify them to deserve a founder's descendant note. Following that, the founding of the settlement is simply history.
My 4th great-grandfather was a UEL (United Empire Loyalist), and is on the official list compiled by the UEL organization. His son, born in Vermont, and who went to Canada as a young child, and as an adult served in the Ontario militia during the War of 1812, is not listed as UEL, even though he is a descendant.
In full disclosure, it is my personal belief that categories are way over used in WikiTree, and that categories should be used only when genealogically meaningful. or can help research. For instance, I might search the category "Mendon: Massachusetts" for families that might have interacted or married into my family who lived there in the early 1700s. On the other hand, the category "Left-handed pipe fitters" really is meaningless for genealogy research, and should not be here on WT.