A view from someone who had to ask for a phone number as part of their job. Who cares!!!! Give a false number. The ones that had to protest or add more drama to the cashier's life are taking a stand by doing what??? The cashier who is there to collect her paycheck is only doing what is asked. Chances are she is not going to speak directly to the owner of a chain to voice concern about phone numbers. Zip codes always blew my mind. How on earth is someone going to find you by your zip code????? I sometimes would plug in a phone number just so I would not have to deal with the "why."
Why? Years ago, (long story short) I met a guy at a store who was handsome and flirtatious, but gave me the willies. Because I was filling out a store form with him, I had to give my info. He kept commando flirting, despite many reminders I was married. Finally, I had to get snippy with him and he went from Dr. Jekyll to Mr. Hyde in record time.
After that, whenever I went in that store, he would shadow me. Gave me the creeps.
He often met women via his job and lo and behold, he had my info. He had even lived in my neighborhood at that time. Worst of all, he had a specific ""ideal woman" and I fit it to a T. I may not be every man's dream, but I was his. 
Whenever I go to a store that asks for my number before they check me out, I *politely* tell them "Sorry, I don't give my number out" and at times it'll send the cashier into a tizzy.

Once in awhile I'll get a giggle out of the cashier for that one