According to the Safeway website,
The “just for u” program and “Personalized Prices” (“the Programs”) are intended for the sole use of the Safeway Club Card-associated household. A violation of this policy may result in our taking action we deem appropriate, including without limitation, issuing a warning and suspending or terminating your access to and use of the Programs. Source
I haven’t shopped in a Safeway in years, so I don’t know how strict they are about policing this policy. My experience in other grocery chains is that the check-out clerks don’t give a hoot who owns the account. They ask if you have a membership. If the answer is ‘yes’ but the card or tag isn’t in possession, they ask for the phone number associated with the account. Once, the person in line behind me offered his card. There was no benefit in his doing so other than a discount to my bill. The clerk didn’t bat an eye while accepting it. Sometimes, a clerk will enter their own code. I don’t know if this provides them any benefit or not.
The point is, while it may be against the rules, the grocery store clerks do not seem to care about adhering to the policy. Maybe they do it out of empathy for customers who would otherwise not be privy to a discount. Maybe they gain a benefit from using their own number. It could be that they are savvy enough to recognize that this gesture generates customer loyalty.