They were given weapons! The PA "police force" was armed and trained by Israel and the CIA under Oslo. Part of the PA police force duties included confiscating arms and disarming the militias (Annex I, Art. II, 1 and art. XI). The PA abrogated the Interim Agreement and expanded the police force beyond the permissible limits (now around 40,000 - 50,000). They were supposed to extradite terrorists, arrest them, etc. They had some show arrests shortly after Oslo but that was the extent of it. The militias are now stronger and better armed then ever. What a failure. Of course, it's obvious they had no desire to carry out any of these stipulations. Just another tactic of ruse and attrition by the PA leadership against Israel.
Whose country is it, anyway?
If it's the Israeli's, then they've got to do the dirty work and at the same time abide by the rules of law regarding criminal justice the rest of the Western world adheres to.
If it's the Palestinians, give them their own state.
You can't have it both ways. If you want the Palestinians to get rid of the terrorists themselves, to benefit Israeli Jews, then the Palestinians are going to need some positive incentives to compensate them for this difficult task.