Q. If I submit a query to a publication outlining an idea I have for article, how do I know the editor won't just steal my idea and give it to another writer to write?
A. You don't. But in all my years in this business, I have never heard of that actually happening. As long as you are capable of writing the piece, there is no reason for the editor to assign it to someone else. Besides, most editors have more than enough ideas coming in that they have no reason to steal ideas. Risk to their reputation would also be on the line.
There is one scenario that could happen along these lines. Sometimes an author will query on an important idea, but the editor (for various reasons) may think that author does not have the necessary skill to write it, or perhaps has not dug deep enough to get the whole story, so they will actually buy the idea from the original writer and assign it to someone more qualified.
It is not unusual to hear a writer complain that their ideas have been stolen, because an article on the same topic came out shortly after their query was rejected. The reality is that a lot of writers do submit articles on a similar topic. Since those topic are often tied to what is going on in the world, it is not unusual that more than one writer would have the same idea. I've been telling writers for years that when God inspires a writer to write on a certain topic, He can't trust them to actually follow through and write it, so He inspires other writers to write the same thing.