Very interesting. There is almost always a price to pay for speaking your mind. Unlike some others who commented, I don’t necessarily feel like something is wrong with the company for not wanting to hire someone who might write about them next. It’s their right to do so. It doesn’t mean the company has something to hide or that they feel threatened. Even if you leave out names etc. it wouldn’t take a lot of work to figure out what company you were talking about if you have a Linkedin page etc. And it could easily be seen as a writer profiting off his employment if he writes about work-related experiences after work.
I am really glad you wrote about this though because it does bring up the fact that there is a price to pay for writing. But that doesn’t mean it’s not worth it.