My favourite (mostly because I know the details and personally trust the source) was for a cashier position in a small grocery store in a small city. When the applicant tried to negotiate on the terms, they were told that someone could just walk away with their supplier information. While it may have been a legitimate grievance, it was a poor excuse to harm someone's future employment prospects. (Small city == limited choice.)
> It seems a bit silly to me, as it seems to cost the employee more than the benefit the employer receives from that clause.
Are you surprised employers can exploit unskilled laborers? It's not silly. It's asymmetrical power resulting in exploitation of people who don't have many options in jobs they can find.
> For the unskilled labourer, it doesn't matter much whether they work for Burger King or McDonald's.
It also doesn't matter for those companies which cashier out of the 100s of applicants they hire. Your only power as an employee is the power to quit and if you can't guarantee that you'll get a new job quickly enough, you're powerless.
You’re assuming a extremely tight labor market, which may be true since about a year ago, but definitely has not been true for at least a decade before that.
Before the current tight labor market, if someone who was living paycheck to paycheck found themself unemployed, they took whatever job they could get. Maybe they apply for 10 jobs and only hear back from 1 for an interview. They have hungry children to feed and don’t have any money for groceries. The employer understands that most applicants are desperate for any job so can tack on any terms they want - such as a non-compete.
Here is Wikipedia article on non compete clauses. Seems like they are quite common everywhere with various restrictions. Amazingly North Dakota and Oklahoma also have a total ban in non-completes along with California.