Netflix wants you to stop getting the milk for free and buy the cow.
Some users will be asked to prove they are authorized to use the account.
According to the Netflix Terms of Service (which we all read, I’m sure wink wink) The Netflix service and any content viewed through the service are for your personal and non-commercial use only and may not be shared with individuals beyond your household. During your Netflix membership we grant you a limited, non-exclusive, non-transferable right to access the Netflix service and view Netflix content.
Netflix officials say some users will get a pop-up when they sign-in asking them to verify that they are authorized to use the account by way of an email or text sent to the account owner. If they can’t confirm, they will be asked to sign-up for their own account (presumably loosing access to the one they were using)
At this stage, account authorization pop-ups are just a test and only a limited amount of users will receive them.