Well, assuming Reddit executives were telling the truth about their goals/needs, which I don't think they are.
They claim the purpose was some kind of emergency band-aid to stop the service from hemorrhaging cash from evil large-scale data-sucking AI developers without compensation... But in that case, they could have simply introduced it as a fresh terms-of-service restriction, with some payment-tier to come later that permits that use of the data.
But AI developers/companies seem to almost universally believe they have fair-use rights to train their models on any data they can get their hands on, and a sufficiently expensive API at least forces them to do engineering work to get all the data. So at the time I believed Reddit's reasoning.