Pretty sure that's not "stealing".
Aside from the intent of the purchase, there's really nothing wrong with that. The mail was sent to them, after all, and without the intent it could easily just be an accident.
Regardless of the researchers' intentions, somebody has a responsibility to address their mail correctly. I get enough email from my namesakes to (first).(last)@gmail.com to know it's not the recipient's responsibility.
These researchers fully intended and expected to capture some e-mails. They didn't want to steal them, they weren't doing anything wrong, they just wanted to see how many e-mails they'd get and write a paper. I'd say intercepting the e-mails is at least a better description than theft.