Brig.-Gen. Muhammad Masri of the Palestinian General Intelligence Service says that the PA has the capability to reign in Hamas, but sees no political reason to do so (report).

Some quotes:

"Capabilities and principle are two different things. Besides ending the occupation, our major goal is not to be labeled collaborators," said Masri.
...

"Why should we be responsible for security in Tel Aviv," asked Masri, "when we have enough trouble protecting our own people against Israel?" For this reason, the GIS is not engaged in roundups of suspects, but is attempting to dialogue with them.