Then it would be possible to actually identify suspicious behaviour, and you could publish stats about companies' hiring practices so candidates can avoid them etc.
The alternative could be like a $2000 fine per listing violation. To make it worthwhile to enforce, offer half the fine as a tax credit that can be claimed anonymously after an investigation.
The point is if a company fills 80% of its job postings with internal hires then that's highly suss and can be investigated. I don't delaying advertising would change that?
1) Person believes company is posting fake job listings and notes a few as evidence.
2) Person submits this to some government form similar to an FCC complaint form.
3) Government contacts the company to investigate or whatever.