[IANAL]
In the US, ownership of real property tends to be more complex than that, and that's what really drives the need for clear title.
The conveyance of real property is further complicated because it is regulated primarily at the state level and there are many and varied laws among the states.
In general, due to their heritage in English Common Law and origin at a time when land was granted by the sovereign, real property is conveyed with all previous restrictions on its use and disposal intact [except in such cases where the restrictions are illegal, such as prohibitions on the sale of the property to persons of particular races, religions, etc.].
This means that a violation of a particular covenant [deed restriction] gives the person conveying title [or their heirs or assigns] the right to take back the title. It is misleading to think of the ownership of real property as consistent with Lockian notions of private property. Restrictions on digital media and software licenses are probably better analogies, and it may be worth noting that these restrictions are not unprecedented in so far as real-property law provides precedent.
Ultimately, the title to real property is never clear - you can't prove that no claims exist only that none have been found.
And that's what's motivating the purported goings on in Franklin County. For all the years they were locked up, those records were not included in any title search. The existence and use of such records would undoubtedly slow the sale of real property in the area. Furthermore, the likelihood that claims against title could be brought forth against current title holders increases significantly.
Given that the records date from the antebellum period, the 800 pound gorilla is not so much the descendants of former slaves as Native Americans. North Carolina was only a generation or so off the frontier and less than that from the Trail of Tears, and while the heirs and assigns of individual former slaves are fragmented, the heirs and assigns of the various Native American nations such as the Cherokee are organized and have legal standing under Federal law to challenge property title. [1]
[1] In the 1980's the Seminole Tribe acquired a reservation that was subsequently developed into Tampa's Hard Rock Casino in exchange for giving up a claim upon land in Tampa's Central Business District where a Native American gravesite was unearthed. http://tbo.com/list/news-opinion-commentary/from-graves-to-o...