As I understand it, signing bonuses started out as upfront payments that would be negotiated to compensate top executives for taking the risk of leaving their current job.
Using this for warehouse workers sounds like little more than a tool for recruiters to mislead people before they’ve read the terms of the contract.
Except after taking that risk, if the exec leaves within a period, they have to return that bonus. So for the exec as well, it’s both inducement and retention.