Why is it a sin to talk to the dead?

Jesus spoke to his dead - Moses for one and if the bible is to be believed - Lazarus for another. Jesus also said - do as I do - as my father does.... (or something along those lines.) so why is it evil for the living to talk to the dead. and where in the bible does God say..."Its evil to talk to the dead". I recall there being a peage which states "Leave the dead to God" but I can not recall anywhere in the bible that says "talking to the dead is evil". Its not even covered as a mortal sin like - "thy shalt not covet thy neighbours wife"... no guessing please - I want biblical quotes.