You’re right — it’s so frustrating. I’ve had the same experience, where people helped themselves to my writing and only added a credit after I emailed.
The only way I can think of for writers to protect themselves is to come up with some unique word pairing that you include in every article somewhere, and then set up a google alert for that phrase. Sounds kind of ridiculous, but it would probably work somewhat decently. Most people who steal content are just scraping content that ranks well and don’t check too carefully.