Previously, players were able to use Station Cash (SOE's universal game currency) to snap up single-month subscriptions. This allowed frugal players to acquire discounted subscriptions, as SOE occasionally has a sale on Station Cash and throws in-game deals for spending it.
Community Manager Piestro confirmed the removal in a forum post: "Hey folks, just jumping in to let you know this was indeed an intentional change. This decision was partially based on the very limited utilization of this feature."
This removal comes at a bad time for players who did purchase subs this way, as SOE is giving bonus Station Cash to players redeeming game cards this weekend.
[Thanks to Don for the tip!]