Why did the senate vote for immunity on the Felony acts the President ordered in violation of FISA?

I cant understand why they would cave in on the 4th amendment. The telecoms were asked to break the law they knew was illegal. FISA was written for this exact reason. Why couldnt the Bush admin just tap the lines then get a secret warrant from a secret court as the law requires? I feel that Obama made a huge mistake by doing this. Do you agree?