US federal employees are now guaranteed all back pay. While that doesn't justify having to show up without pay - at least Congress has passed a law, signed by the President, that guarantees all workers will get back pay.
The 2019 Government Employee Fair Treatment Act ensures back pay for furloughed workers and mandates that the roughly 500,000 employees working without immediate pay during the shutdown are able to take previously scheduled leave without consequence.
As an aside, US federal employees are prohibited by law from going on strike (Taft-Hartley Act) - which is why you see some "calling out sick".