Yep - I work on an email platform and tracking pixels are common.
However, the ones I work on only record the time that a given email was opened ... BY DESIGN they do not include WHERE that email was opened, the IP address of the reader or anything else, because a) that's just creepy and b) it's not necessary to measure the impact of a given campaign.
But frequently requested/demanded/wanted by email marketers? Yep.
And, perhaps surprisingly, open tracking (when used effectively) serves to reduce unwanted email.
Say you run a really popular store--tons of loyal customers who genuinely want to know about your sales/specials/coupons/whatever, tons of different marketing channels. When you send an email campaign, you want to be able to tell whether increased traffic is due to that campaign versus some other marketing, so you track link clicks (fingerprinting each link in the email). Presumably this isn't generally considered unethical (if it is, is server-side request logging problematic as well?).
You, the hypothetical marketer, want to know which category your campaign falls into for your recipients: "hate it" (marked as spam, you usually get notification of that from the mailbox provider), "don't care about it" (never opened it/filtered it into purgatory/deleted it), "maybe care about it" (opened it but didn't click), or "actually want it" (opened it, clicked on something).
Open tracking is important to disambiguate "don't care about it (and might start marking as spam if they get another email from me)" from "was interested, but not enough to click on a link". If there are no clicks, and really low open rates, it's a sign to stop emailing those people immediately, or else they might start marking the email as spam and get your sender reputation penalized, which is very bad for business. If there are no clicks and good open rates, it's a sign that people are interested, but your content might suck/have display issues/be poorly put together and need to be improved.
Because of this dynamic, being able to track opens with reasonable fidelity (sure, people can block images or spoof opens if they want) is key to reducing spam and low-quality marketing content in many cases. Businesses suffer pretty directly and immediately if they don't back off from sending tons of crap to non-openers.
I know this probably seems alien to a lot of the HN email-using demographic, but there are large swaths of people that very much do use marketing email heavily to stay informed and buy things, and they willingly agree to receive a lot of it.
This isn't a blanket statement that "all activity tracking is inherently ethical" or anything, just that, in this case (and given that email marketing and facilitators thereof aren't going anywhere) this particular kind of tracking, while sometimes ethically dubious, has a bright side.
Sending email has a cost, however small, and it's better to send things people engage in and enjoy ... which is why knowing click through and open rates is useful.
The people you're sending these tracking pixels to should be the final arbitrators. So are you willing to have them reply yes/no to a form that says: "Do you wish to transmit the time at which you opened this email to $marketing_company". If not, why not?
You broke the site guidelines repeatedly in this thread. We ban accounts that do that, regardless of how right you are or how wrong other people are or you feel they are. Would you mind reviewing the site guidelines and sticking to the rules when posting here?
However, the ones I work on only record the time that a given email was opened ... BY DESIGN they do not include WHERE that email was opened, the IP address of the reader or anything else, because a) that's just creepy and b) it's not necessary to measure the impact of a given campaign.