They'll pay it when they take it back out. At best they're saving the difference between the bracket rates in exchange for letting your money slosh around the markets for your working life.
500k in income 1 year is ~$162k in income tax.
500k in income split across 2 years is $68.5k each year totalling $137k.
It's not massive but it's still a decent chunk of taxes people can avoid.