I'm gonna take some guesses:
1) The land still has value to NS - short term via the sale of track, long term via property lease to the track owner and/or other utilities. Depending on the state, that arrangement may have an impact on the assessed value for property tax.
2) As the underlying property owner of record, and with deep pockets, they're protecting themselves somewhat from any legal actions against them due to injury or accident.

Them's my theories.

I'm guessing that part of it is the legality of the granting of the ROW easement, perhaps under eminent domain, that would require the owning entity to be a 'railroad'. The woning railroad could then lease the ROW for any other purpose -- cable, fiber, pipelines, etc. but also shortline or even museum trains -- as long as they maintained the ability for the track to be used by trains.

Waiting with some interest to see what happens on the 'trail' part of the Adirondack line...

some opinion, and some fact...

1) If NS retains ANYTHING, property rights, operating rights, whatever, on a line through agreement or lease, that lets them control the liability insurance limits. There are some fortunate few grandfathered in, but for the most part, you're going to need a friend in Lloyds of London to clear a liability policy in this age. (fact) along with all the NS leased line agreements I've reviewed specifically prohibit passenger operations.

2) Much of the lease vs. sell issues with NS seem to have originated from the Conrail lines takeover, where they had to apportion the total purchase price to.... something. To keep everybody happy I've always felt that the reputed 'overpayment' was heavily apportioned back to real estate (rather than accounting "goodwill"), which means if you sell it you book a loss on actual current value. No happy shareholders. So the preference to lease to shortline operators, particularly on ex-Conrail territory, has continued. (opinon only).

I can say that, with the one line in question, retention of the right-of-way makes no logistical or strategic sense for NS, as it's a dead-end spur, has only two customers left plus a spur to a power plant that no longer needs any rail service whatsoever (converted from coal to gas and solar, and the new transformers and turbines went in BY ROAD), and includes one massive liability of a half-mile-long 135-year-old steel truss bridge.

All the experts I've talked to, including people involved in the negotiations with NS, just shrugged their shoulders, or engaged in anti-long-term-liability-risk "conspiracy theories" akin to that suggested by Brother Gustafson........ though the accounting proposition theory seems intriguing..............

Isn't/ wasn't the Roosevelt Railroad Museum nearby? I believe they owned a section of the CofG line toward Columbus, GA.

They own about 8 miles of the Southern M-Line on the other side of Griffin. It crosses the Central main in Experiment (North Griffin) just north of the spot where the line in question diverged to the west before the switch was removed.

That's what I thought. If the group in the OP, claiming to want to operate steam in the area, is legit (I have my doubts) the moribund Roosevelt RM line (a reference to FDR's "Little White House" nearby) would seem a lot easier to get access to than anything owned by NS. And I believe the line is still in place to Columbus, albeit with trees growing in the gauge.

Sadly it's been gone from Rover (just SW of Griffin) all the way to Columbus since around '91. The last train was in '88. Gone like pulled up and abandoned though there's a rumor that NS owns some of the property down through there and along the ROW that diverged at Williamson (a few miles from current EOT) and ran to Ft Valley. I have never investigated the validity of said rumors. Griffin was a busling RR town up into the late 70's and early 80's. In addition to the Central road trains and locals that are all that's left today, there were turns to Columbus and FV along the M and FV lines, a Griffin-Cedartown job on the C line, and tons of local switching for the textile mills.

I thought that for some time NS wasn't even going to sell lines, just lease them, then a few WATCO/RJC lines popped up that I think are owned. I remember when CSX made a huge deal about selling off a massive portion of lines but it never really happened. Seems that ship has sailed to be able to buy a line from a C1 unless you are a WATCO, RJC etc, already"in the club" so to speak.