I doubt efficiency is their primary concern. It seems that it is more likely to be efficacy that concerns them most. Namely, does it generate more income than it would without it?

The answer to that is probably yes. After all, it isn't their resources. They are simply adding a line of code to their markup and calling it a day.

I see no problem with this so long as they get consent and, ideally, allow a method to opt out, though the back button is opting out. I'd also mention that it should be informed consent. In plain language, the user should be aware of what is happening.