> * The worker gets to leave their sponsoring employer on the day of their arrival, if they choose to. The employment contract may not include any clawbacks of anything.

This would be workable if it also results in the person losing their visa. There must be some downside for the employee, otherwise it's an invitation for abuse.

If the worker gets to keep their visa then it's just a backdoor way to get a company to pay for their visa and relocation so they can immediately quit and then go do some other job they actually want (at no expense to the next employer).

The final scenario you describe already happens with immigrant visas. Once you have your Green Card you are free to quit the sponsoring employer and work for whoever you want.