Then it should be clear in the recruitment process that working on weekends is non-negotiable so people who can't won't apply for the position.

That shouldn't be a secret feature, they are wasting their own time and resources by hiding it to applicants.

Oh I dunno, maybe if you ace the interview they may skip that. Certain senior engineers who are in huge demand don't need to do such things. They have folks lining up to hire them without asking them to do programming assignments. I doubt the filter is used 100% of the time.

Hidden ageism. Hire 22-year-olds with no responsibilities, and not enough experience to know when they're being exploited, is the scam here.

"I suspect that that might be a secret feature."

Yes, agreed. It may also test, if you are really serious with your application. If you are, there will be a way to free some time.

This goes both ways: companies blow time to interview candidates, who just want to see how far they get. Or who plan to negotiate better terms with their current employer (happened to us recently, we paid flight, hotel, lost 3 months and a whole application round).

Candidates blow their time by implementing stuff for companies, who just want cheap code or who want the best solution for a problem they cannot solve, or simply re-decide their plans after you invested your time for the coding.

Both sides should find out how serious the others are. Payment for code is a good step. But if you pay, you can also ask for something, right?

s/serious/desperate/

Is your "application round" limited to a single candidate?

No, we had a couple, but after the one we offered the job to finally let us know he wouldn't come, the whole shortlist had already found other opportunities.