They pay the user for it with increased Google Drive space, a free subscription to the NYT, etc.
I did out of curiosity after I answered a few questions, and sincerely regretted it. The notifications and junk e-mail were never-ending until I blocked them all. And my time is not worth that little.
You may not have to stay subscribed, but that's true in a sense for most junk mail.
Some Google products are stupidly designed, but this one looks like it's doing the right thing.