Unfortunately, most countries don't have, or don't enforce, penalties for companies claiming rights they don't have. So that's the first part: Companies claim, often using vague or misleading wording, to have more rights than they can legally claim.
And what they can claim varies but can go quite far. For example, even employee-friendly California with a law specifically to limit far reaching clauses (https://law.justia.com/codes/california/2011/lab/division-3/...) allows terms in employment agreement that assign to the employer the ownership of "invention[s] that the employee developed entirely on his or her own time without using the employers equipment, supplies, facilities, or trade secret information" as long as they "Relate ... to the employers business, or actual or demonstrably anticipated research or development of the employer". For a megacorp, that can be basically everything.
The second part is that in some countries you can be fired for any reason, or no reason, except for specific protected reasons. "We didn't like your blog post and are worried that you might accidentally reveal company secrets in one of them in the future, so we're letting you go" would be legal in many places. "All social media or personal web site content has to be reviewed to make sure you're not leaking company secrets" is also a demand some companies make.
Likewise "we want you to give 100% at work and your side project clearly shows you're spending too much time elsewhere so we don't believe you're working to your full potential", even though stupid, would be a perfectly legal justification to fire someone in many countries. Or just the manager silently thinking that and silently retaliating through e.g. denying promotions.
Are you using "many countries" to mean "many US countries"? What you're describing is rather unheard of here in Europe. I wouldn't even call that California example anywhere near "employee friendly", it's outright ridiculous.
What does a job have to do with publishing hobby projects?