So that the Palestinians would agree to talk. That's the deal Israel had to agree to just to start the talks.
Why agree to it? Because Bibi couldn't afford to piss off Obama any more than he already had, so he had to cave in on the precondition (which no one wants to call a precondition, just a good will gesture).
From The Times of Israel,
Israel publishes list of Palestinian inmates to be freed:
A panel of Israeli cabinet ministers on Saturday night approved a list of 26 Palestinian prisoners to be released on Monday in the third phase of a four-stage series of releases agreed on when peace talks with the Palestinians were resumed earlier in 2013.
All of the prisoners on the list (Hebrew PDF), save three, were convicted of murdering Israeli civilians or soldiers, as well as Palestinians suspected of collaborating with Israel. In a press release Saturday night, the Prime Minister’s Office said all of the inmates had been convicted of offenses committed prior to the signing of the Oslo Accords in 1994....
Among the victims was a Hebrew University Professor:
•Muammar Ata Mahmoud Mahmoud and Salah Khalil Ahmad Ibrahim, convicted of murdering Menahem Stern, a history professor at Hebrew University. Stern, 64, a winner of the prestigious Israel Prize, was stabbed to death while walking to work at the university’s Givat Ram campus on June 22, 1989. A monument in his memory figures in a scene from the prize-winning Israeli film “Footnote.” Ibrahim was also convicted in the murder of Eli Amsalem. In addition, the two murdered a Palestinian suspected of collaborating with Israel, Hassin Zaid.
More on Professor Stern
here. This image, according to Wikipedia, is of Professor Stern in 1988, the year before he was stabbed to death.
[caption id="attachment_74564" align="alignnone" width="288"]
(Menahem Stern, 1988)[/caption]
There will be celebrations to welcome home these "heroes."
As more information becomes available as to the victims, we will update this post.
Update: Israel National News (via Carl in Jerusalem) has more details: