Actually, Google isn't alone either. They have also combined forces with the Electronic Frontier Foundation, TRUSTe and others who aim to ward off prosecutorial requests insistent upon revealing emails. Perhaps not so coincidentally, a number of the organizations involved also happen to be members of the Digital Due Process Coalition, a group dedicated to improving digital privacy laws and practices.
Yahoo's camp argues that people have the reasonable expectation of privacy where electronic mail is concerned. Because of this expectation, they suggest a warrant should be absolutely required to hand over personal emails -- a point the Digital Due Process Coalition has been driving home for some time now. Their explanation is that warrantless email seizure is a form of unreasonable search, thought to be prohibited by the Bill of Rights' fourth amendment.
Another notable point made is that the laws which govern digital privacy do not necessarily address the technology of today. A loophole in the Electronic Communications Privacy Act of 1986 is cited as an example, which potentially allows government agencies legal access to otherwise private data stored on remote servers. Email, in fact, is a prime example.