It makes less sense in the day of computers and the internet, but I think the reasoning seems to be that law enforcement asking someone that you voluntarily gave information to, for that information, is not a search of you or your property in the 4th amendment sense. So if the that someone gives that info to law enforcement, there's nothing you can really do to claim they got that info by unreasonably violating your privacy.
Or, to put it another way, the government needs a warrant to prove searching through your stuff is justified, but you cannot compel others to keep information private on your behalf, or have it count as your information instead of theirs, if they do disclose it.
This made more sense before computers because, by and large, the primary way for third parties to get information that would be considered private would be through voluntary, and considered, action on your part. e.g. your bank holds your banking records
to facilitate your banking actions, or you tell the phone company a phone number so that they can connect you. Law enforcement can then ask the bank for your bank records, and the phone company for the numbers you've dialed. It's not searching you per se, but is rather what the third party has learned from interacting with you.
Today however, thanks to the internet, computers, and cellphones, we practically hemorrhage information into the wider world more or less non-stop with only the barest awareness we are doing so.