Perhaps it has something to do with the fact that Hezbollah won a war two years ago. If Israel wanted those bodies back, that's pretty much the only thing left for them to do. Generally speaking, losers get sore deals when making agreements after wars. As to the imbalance, don't forget that Israel has also cheerfully jailed Palestinians and Lebanese for the sole purpose of using them as bargaining chips in prisoner swaps. Qantar is actually an exceptional case, since he is a genuine terrorist who has genuinely killed genuine civilians -- that's what all the fuss is about.
It's also just barely conceivable -- in light of the Paris conference, the Israeli-Syrian peace talks, the warm Olmert/Abbas handshakes, and even the sudden talk about the US opening an office in Teheran -- that there's some kind of back-room dealing going on about actually making progress on substantial issues. If so, the prisoner thing was something that had to be gotten out of the way, and the only way that would happen is by handing back Samir Qantar.
Now, if Israel has the sense to evacuate the Cheba'a farms, they will have removed the last of the symbolic obstacles to actual discussions on substance about incorporating the Hezbollah militia into the Lebanese army.
Don't forget that prisoner swaps, and deals involving tiny, uninhabited parcels of land like that, have no substantive value -- they won't affect Israel's security or Hezbollah's operational capacity one whit. However, they are of huge symbolic value, and will make it possible to move further. Yes, Israel has lost face with this exchange -- but that's all it has lost. On the other hand, they have a slim but real chance of gaining a great deal in any negotiations that may follow.
I'm no fan of Qantar either, but on this score, Israel did the right thing. With the new political constellation forming there, the Hezb will either talk, or be forced onto the same road as Hamas. They're not stupid, so I believe they'll talk. They may still prove too intransigent to accept any settlement that's acceptable to the Israelis or their Lebanese opponents, but if that happens, at least the Israelis will have tried.