Default Newsletter module sends email when customer is buying some product

[Pekkhyu]

The default module of the newsletter send emails when customers are buyng some product. I don't want this happens.
It must send emails only if the customers decides to subscribe to the newsletter and not when is going to bui something.

At this link: http://helpme.emeraldlab.it/prestashop/bug-newsletter.mp4
you can see the bug

In the attachment you can see my setting for the module. It says that only if the user subscibe it will send the email. 
Indeed, the system doesn't registry the customers as a newsletter subscriber when he buys something.
So, I don't know why it sends the email for the new newsletter subscribers.

Is it a bug? How to fix it?

I hope I explained myself

Thank you,
Pekkhyu

P.S: Prestashop Version 1.7.6

Php version: 7.3

Edited December 18, 2019 by Pekkhyu (see edit history)