Earlier quoted context omitted.
I think any email that uses tracking pixels could be classified as either spam or marketing, or possibly both. If you use tracking pixels in your receipts, I think you're doing it wrong.
What about mandatory service announcements where you want to know if you reached clients before breaking them?
A tracking pixel hit only means that that the email was received and loaded in some email client, not that it was read (in detail, or at all), understood, or acted on.