I believe the "official" answer is that they don't. But they sort of do.
See this interview with David A. Kennedy from the WordPress.com theme team.
We hope to open the form up some time in the future, but don’t have a concrete date. We do invite new shops to WordPress.com when we find themes we love.
Whether our marketplace is officially accepting new theme shops or not, we’re always looking for amazing, new themes. Make one, put it out there, and we’ll probably find it.
If you're interested in being invited as a theme author on WP.com, the best thing you can do is just make a really nice theme. And it wouldn't hurt if you built it on top of Underscores and utilized Jetpack features.
Also, if you haven't already, watch all of the videos embedded in the WordPress Themes and User Expeirence article, and generally just read through every ThemeShaper (this is the official blog of the WordPress.com theme team) article you can to get a better idea of WordPress.com's theme philosophy.