I'm sure this is a common problem faced in competitively set environments, and even more so when the content is a niche topic.
Q: If content is not yet indexed by google is it free for anyone to grab
and copy it to their site?, In the hope that the stolen copy will get
indexed before the original copy? Does this often happen as a black
A: There are many known an unknown factors when it comes to when and how Googlebot does it's crawling. For instance, Google will crawl https first, before http. As the content owner, you do have the sole rights to distribution thereof and as such, should you have a copyright claim, depending on your whereabouts, you may be able to take legal action.
You can consider contacting the webmaster directly and request that the content be taken down, though this may not always prove fruitful. Alternately, you could consider something like Who Is Hosting This to identify who the host is and contact them directly. More than often, the web host will take action in a more pertinent fashion and may even take the website completely offline in order for them to remove the duplicate content.
Another option would be going the Google DMCA (Digital Millennium Copyright Act) complaint route. Taken directly from google.com:
This page will help you get to the right place to report content that
you would like removed from Google's services under applicable laws.
But ultimately, yes this does happen in the wild, you're certainly not the only person experiencing this.
The good news is you have a wide range of tools available at your disposal to help deter this sort of behaviour in future:
Google Alerts: You could for instance put in a portion of your article, choose what types of websites should be searched and then provide your email address so that Google can contact you with the results. (You can set up as many alerts as you'd like and adjust the settings to be notified daily, weekly, or on an "as it happens" basis).
Copyscape : This is a paid for option, but this site will search the web and let you know which sites are reposting your content.
Q: I realize you could use the fetch as google in search console and then
submit to index, usually my content gets indexed instantly when doing
this -- BUT I dont always like to do this, rather preferring google to
discover content naturally. Am I wrong to want natural content
discovery and should I just submit each post manually to the index?
This is more of a subjective question, as there really is no "wrong" or "right" in this instance. At the end of the day, submitting the content directly to Google will prove to get it indexed in a more timely fashion and as a result, have the potential to reduce the chances of you sitting in this situation.
Another great option to consider is posting via Google + which virtually gets indexed instantly.
Ultimately, SEO is a not a destination but a lifelong task which takes constant follow-up and continuous attention. This is simply one of the obstacles that you'll experience along the way. The fact that you've noticed the behaviour already puts you miles ahead of a majority of your competition, so well done!