Jackie has proven herself to be loyal
Jackie had hardly proven herself loyal.
Throughout the whole season it seemed she was on the fence, for example:
- She sides with the party when they initially withdraw support from Frank's presidential bid
- She also leaked hints that the party had withdrawn support from Frank
- She agreed with Dunbar to suspend campaigning due to the hurricane against Franks wishes
- She was initially hesitant to attack Dunbar's kids and Frank had to talk her into it
- Jackie explicitly tried to side with Dunbar but Dunbar turned her down, it's unclear if that got back to Frank
A reoccurring theme is that Frank has shown to be highly vengeful and is rarely loyal to those who haven't been unconditionally loyal to him in the past. He threw her under the bus for all the times that she was on the fence, went behind his back, or made him work for her loyalty.
He claimed to Jackie that it was so it wouldn't appear as though they were teaming up on Dunbar, but his explanation was thinly veiled and Frank most likely expected her to see through it and side with Dunbar.
We'll have to wait until Season 4 to see if that works out for Frank, or if his revenge was ill-timed.