These sorts of tracking pixels or tracking items are long-standing methods of analytics gathering in the email marketing and delivery space. This is not an attack of privacy. They also know every click you make in that email as well. And of course they track the number of times you do these things. They also track that traffic anonymously if you forward the email to a third party who interacts with it.
This is the same practice as any tracking done on a web page even going back to the pixels that AWStats used to use.
I work for an ESP and we've been doing something similar since 2003.
It is an attack on privacy. The recipients don't expect to be tracked in this way, and you don't ask their permission before doing it (1). You're exploiting a weakness in the system.
It doesn't matter how many organisations are doing it, or how long they've been doing it, or how much money they make from it, or how useful the information is, or how good the "services" you provide are. It's still an attack on peoples personal privacy.
I don't expect to be able to convince you though. After all, you work for an ESP so you have to justify it to yourself somehow.
(1) hiding something away in the T&C's doesn't count.
EDIT: Oh, are you talking about tracking someone on your website, or tracking people across websites?
EDIT2: When people visit a website, they expect that website to be able to see their IP address and track their movements across it. What people don't expect is that websites (without their permission) can track their movements across other websites.
This is the same practice as any tracking done on a web page even going back to the pixels that AWStats used to use.
I work for an ESP and we've been doing something similar since 2003.