Seems like a career academic with no experience in the real world playing around like this is some kind of game. I'm sure they meant no harm, because they don't consider anyone "participating" to be anything more than a potential subject in their agenda to get a good review on their paper.
That letter and their social media posts are nothing more than a facade to maximize return with no consideration of impact.
Mayer has a JD and is licensed in CA (I don't know about NJ), has worked for at least one US Senate office, and has been so involved in actual practical privacy work that ad companies pressured the president of Stanford to expel him for his legitimate work on DNT.
The person who designed and ran this study is not Mayer. Mayer runs the lab, but this is a subordinate's baby.
From the study's website: "Please contact the lead researcher for this study, Ross Teixeira (rapt@princeton.edu), if you have any questions, believe you received an email in error, or would like to opt out of any future communication related to the study. The additional members of the study team are Professor Jonathan Mayer at the Princeton University Center for Information Technology Policy, who is the Principal Investigator, and Professor Gunes Acar at the Radboud University Digital Security Group."
That letter and their social media posts are nothing more than a facade to maximize return with no consideration of impact.
Total negligence.