> he thinks it's [...] a legal necessity to protect the company
Exactly, which is why his post is totally self-serving propaganda.
Though he tries to present the post as "here's a totally objective explanation of how side projects interplay with employment law", ultimately his point comes down to Business owners have all these complicated issues to deal with, so just accept the fact that they're going to screw you over, while they make millions of dollars
There's no credibility given to the fact that companies have choices here and there are other options that don't screw over the employees - those options are too hard, and they might introduce legal risk to the company, or (!gasp!) scare away investors.
It's just accepted as given that business owners must do whatever it takes to protect their company's interests and employees should shut up and deal with it. Employees carry the legal and financial risk because it would be inconvenient for the company to do so.
The company is simply acting in its own self-interest and it's not actually trying to screw you (probably), that's just an irrelevant side-effect that they might take advantage of in the future when they do want to screw you.
Now that know that you understand that, you're totally cool with it right? It's just business.
I don't think Joel is intentionally trying to be manipulative. I think he honestly believes all of that. As a business owner he's dealing with all those issues and he thinks he's making appropriate choices and has no particular intention to screw over his employees. The purpose of the post is so that everyone can see that he, and business owners like him, are not a bad guys - they're just acting rationally and you only thought you were getting a bad deal because you didn't understand the choices they're making.