Hello I want to know why the cleavage site of endonucleases are polindromic? Thanks

They are not always palindromic
BtrI, a novel restriction endonuclease, recognises the non-palindromic sequence 5′-CACGTC(-3/-3)-3′ (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=102641)

However if the endonuclease does cleave at a palindromic site then if sticky ends are produced they will be compatible. Whereas if the sequence was not they wouldn't be. Although I can't think why this would have benefited the bacteria which initially evolved them as a defence mechanism against viruses. Good job they did though or science would be a very different place today!

I've had a good look around the web and I can't find a satisfying answer! I'll ask in my lab if anyone knows tomorrow.