The property of a Grasping Weapon allows you to use it with enhancement bonus. "You can use this weapon to grab targets, adding the weapon's enhancement bonus to your grab attack. You can still use this weapon to attack a target you've grabbed with it."
They did however neglect in that to deal with the fact that you can only grab things that are adjacent to you, so by RAW it cannot be used with a grab attack beyond 1 square, even if the weapon has reach.
Then they also neglected to consider ranged polearms and spears when they wrote the weapon's power entry. I REALLY do not think the intention was to be able to have a Grasping Tratnyr and throw it 20 squares and instantly yank the target back next to you, but that is the literal game mechanic as written, no doubt about it.
Personally I'd house rule it. Add a clause to the property which says it allows a grab out to the reach of the weapon, which allows you to pull someone in from 2 squares with it, which I agree was the intent. I would also house rule in the word 'using a melee attack power' in Grasping Weapon's power section, which thus forbids the 'harpoon effect'.
Not that a more limited form of harpoon effect wouldn't be a cool thing for some weapon to have, but I really don't think that was the intent of Grasping Weapon, and having it work out to a range of as much as 20 squares is a bit extreme. The purpose of Grasping Weapon was to make grab a viable attack strategy. Currently grab is pretty much worthless much past around 8th level because it is a non-weapon and non-implement attack, thus it cannot get enough of a bonus to keep up with monster's increasing defenses. By epic tier it is thoroughly worthless, as is bullrush as well. There are a few other things like Alchemical Items from AV that suffer basically the same problem, though they hacked in a solution there by just hardcoding in a scaling +X vs Reflex to-hit (something nothing else in the game has). Grasping Weapon is the 'hack' to let grasp work at high levels.