This is just another illustration of how behind in technology the law is.
Under the third party doctrine, any information exposed to third parties is not subject to 4th amendment protection. Basically, the Supreme Court held that the government did not need a warrant to obtain the phone numbers people dialed, because they had to reveal the phone number to the phone company.
That has evolved into, the 4th amendment does not protect any information you put online (including email), because you expose it to your ISP or other service providers.
Kagan indicated she'd be willing to scrap that rule in her recent concurrence in the GPS tracking case, however, it hasn't happened yet. Until it does, this bill is entirely constitutional...