>Now say John travels to another town, and the proprietor of a similar establishment in that town, wanting to provide John with the best level of service, calls you to ask "Hey, what does John like?", and you tell them.
That's unacceptable to start with.
>What has changed exactly?
You've automatized something unacceptable ?
>Don't you have a right to record who transacts with you?
Within a certain context, you do. The data that is untrusted to you is done so based on the assumption that you're acting in good faith and won't trade that information without consent.
>Isn't that information yours, to do with as you please?
Absolutely not, at least not in the legal systems we have in Europe.
>Can "John" command us to forget we ever saw him?
Yes, he can ! At least in my opinion and virtually every other European's he does. The right to be forgotten is an active subject of discussion[0]. The stance in the US is that it runs contrary to freedom of expression. The stance in Europe is that personal freedom implies being sovereign over one's own data. In technical areas, the right to be forgotten is interpreted as the right to erasure[1], which happens to be part of the GDPR. I've myself used that right several times. And I end up being very mindful of my usage of data in software I write.
>How is any of this reasonable?
Your ease of business doesn't trump someone's rights over themselves, the data they generate being a extension of it. End of story.
[0]: https://en.wikipedia.org/wiki/Right_to_be_forgotten
[1]: https://gdpr-info.eu/art-17-gdpr/