Actually, no, they don't. I own a cellular phone store. Since early 2008, you cannot use your last 4 of your SSN for identification, you have to create a separate 6-10 digit PIN to access your account with Sprint. Contracts that must be retained by both the carrier and the store must be held on file for the length of your contract, which is typically 2 years. This contains no personal information aside from name/address. The application which contains information must be held for 30 days, and then destroyed. Sprint is one of the better carriers for this, actually. They will audit locations about once a month to check that the proper documents are stored and shredded.
Also, should you wish to get a cellular account without supplying your social security number, the individual at the store should have been able to do it for you. It is not a difficult process, but will require you to place the carrier's maximum deposit limit, typically $400-$500. You were simply at the wrong store, with uninformed people.