My mistake, I thought an OA was a reaction not an interrupt. In any case, Polearm Gamble says, "When a nonadjacent enemy enters a square a adjacent to you, you can make an OA..." The enemy is already in the square adjacent when the OA happens.
I'm not saying every encounter should be 100% ranged. I'm saying that the counter to melee beasts is to hit them from range - not just by the rules (the melee beast is not very effective at range), but also in real life. A smart person doesn't engage the master swordsman wielding two scimitars in melee, he shoots the swordsman with his revolver. If the party is facing only melee opponents all the time, that to me is artificial and metagamey (unless they're all unintelligent creatures). And it has nothing to do with the feat!It's metagaming to start arming your NPCs with bows in a different proportion because of the feat choice of a PC.