The spines do not become "blunted" on all varieties. Its truly like a mango, one needs to know the proper stage to harvest a jackfruit for eating as a ripe dessert.
As stated, many are picked green/immature/not fully ripe and could be done so for multiple reasons which most common are being shipped and sold commercially, picked for eating as a vegetable, and because people pick having no clue as to when the fruit is actually ripe.
In my opinion, and some won't like what I say, but buying a jackfruit in a location where they are not farmed/grown is less than a crapshoot. You have little chance if buying anything decent or ripe. I know some will say they have bought them in Idaho from a market and they tasted delicious...I would challenge that and say you should try one properly picked where they are grown.