If a participant specifies a 100%-of-pay elective deferral, many employers and plan administrators interpret a plan to restrain such a deferral to what’s available after required withholding.
Here’s a simplified illustration, assuming no tax beyond Federal taxes, and assuming nothing else taken from the worker’s pay:
Pay before any withholding $10,000
Withholding for OASDI tax $620
Withholding for HI tax $145
Net pay after withholding for wage taxes $9,235
Federal income tax withholding (22% x $765) $168.30
Pay available for elective deferral $9,066.70
Elective deferral $9,066.70
Net pay $0.00
I’m not aware of a particular statutory authority. Some might follow a principle that what otherwise would be a contract promise or obligation that would require a person to disobey applicable public law is legally unenforceable.
This is not advice to anyone.