Usually signing bonuses are "pay back all or a pro-rated percentage if you leave before [date]"
Sure, but that's just lending you a computer at that point, unless they expect you to cover the depreciation cost or something when returning it.
I would guess you would owe some agreed value of the PC at the time you received it.
From what I've seen where I work the majority of "work" computers and laptops don't get returned if the employee separates for whatever reason. So I can see some companies wanting to make it a formal loan or have some means to recover the lost value if it isn't returned. My employer doesn't, because the residual value of a 2-3 year old laptop is not even worth anyone's time beyond making a few perfunctory requests to return it.
Eh, the requirement to return your work-provided laptop is about as strong, legally speaking, as a requirement to pay back the value of it when you leave. Perhaps even stronger (because a work laptop is never really even a loan to you). I would suggest your workplace is a bit of an outlier: most places I'm aware of make pretty sure the laptop is returned, even if only because of the risk of company information leaking out otherwise (some places I know might let an employee keep an old laptop that's being replaced after they have wiped it, perhaps with all storage removed entirely).
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That's exactly how those things normally work.
"Education incentives" at companies can be similar.
For instance, maybe they pay for your trip to AWS Re:Invent and a couple of cert fees ($X,000.00) with the agreement you stay for a year or two. As long as you stay for the year or two, you don't own them any money. If you leave before then, they send you a prorated bill or take it out of your last paycheck.
The idea is that companies don't want to train you on their dime for you to use that training to immediately take a new job somewhere else (or worse, a competitor).