A copyleft license would not preclude this, as the Atom authors would still hold copyright, and could freely distribute a "Pro" version under a proprietary license.
A copyleft license would prevent someone else, who does not hold copyright on Atom, from doing that.
So you're happy that the original authors use copyleft+proprietary instead of allowing anyone else to release a fork (which would also be free software)? How's that better?
This is called "open core" strategy and few companies use it correctly because there is so much incentive to focus development on the "pro" version and let the free/open one stagnate (or lack basic features).
I wish they had it released under a less restrictive license like BSD or MIT.
Well, it's also very difficult to justify contributing to a project that's open core because it means assigning your copyright to someone who stands to profit from your work while denying you the same right. There have been projects that have managed to still get contributions under this model [1], but they're probably the exception rather than the rule.
Of course, then when you don't get any contributions that only feeds the justification for focusing on the paid product, because clearly "the community doesn't care".
[1] mysql is the biggest and most obvious example, but it seems like a lot of tension built up over it and that's resulted in several forks existing now. Who presumably accept contributions without attribution to Oracle.
A copyleft license would prevent someone else, who does not hold copyright on Atom, from doing that.