I suspect
@vgrinshpun is correct. Although Tesla may contract with Samsung (or Panasonic via partnership) to manufacture them, I suspect the intellectual property regarding them (chemistry, anode/cathode design, case design, foregoing cell-level safety features, etc...) remains Tesla's. As such, while we can buy cells from thoe manufacturers, they aren't necessarily the same.
It's long been widely held that the Panasonic NCR18650B or NCR18650G cells are
close to Tesla's, but not necessarily exact.
Lots of companies hold IP and contract out manufacture. To use the favorite examples: Foxconn manufactures for Apple, and you and I can buy from Foxconn, but we can't buy the exact parts to make an iPhone.