I previously worked on couple of companies that flirted with the line on some of these dark patterns. As far as they go, this one is pretty benign. If I recall correctly, I was for doing this pattern. There is enough intent there (putting your whole email into the user form) that it really doesn't feel like abusing the user. That being said, me and my coworker had to fight back and refuse to build a few other dark patterns.
The real issue is the legitimate companies that take it way past dark patterns. Credit rating bureaus that scrape 100% of your data and the data of anyone you've ever been near, Facebook stalking you across the internet, etc. As far as I'm concerned, if it's okay for Facebook to do this exact dark pattern on everyone in the world, I can make a stupid directory site have a little more juice.
I don't see how this is benign in any sense, but I may be missing something. If I type my email into a box on a website ...and decide a moment later NOT to press "Submit". My expectation is that my email was NOT saved to the server. I didn't consent.
If it's "pretty benign" to save a user's email before they click submit, then it wouldn't be a big deal to inform the user that you just saved their email instead of trying to silently hide the fact.
The real issue is the legitimate companies that take it way past dark patterns. Credit rating bureaus that scrape 100% of your data and the data of anyone you've ever been near, Facebook stalking you across the internet, etc. As far as I'm concerned, if it's okay for Facebook to do this exact dark pattern on everyone in the world, I can make a stupid directory site have a little more juice.