It's actually a good/complicated question. I don't know the answer for Kaiser, but this comes up for residents getting jobs after graduation.
It turns out that actually giving you a bonus when you "sign" is complicated. Bonus is salary, so they have to have all of your tax information, withholdings, pay their half of FICA, etc. And then, if for some reason you don't start, they then have to decide whether they want to try to get their bonus back.
Often, the "signing" bonus comes in your first paycheck. It's much easier that way.
Once, I reviewed a contract where the signing bonus was an interest free loan, that then was forgiven over a period of time. That's a really clever way of doing it, as a loan is a tax-free event, and then the forgiveness triggers taxes (but they can then withhold those), and if you somehow don't end up starting work, they have you on the hook for repayment because it's a loan.
