Nothing stops the workers from buying bitcoin as long as they don't buy it in their offices which maybe have cameras, so Bitcoin is a better alternative to Banks and Western Union, as everything can be controlled from your house PC desktop or hardware wallet for more safety. You are your own bank with bitcoins, so yeah nothing stops these workers to buy bitcoin in the end if they wish to do so.
They cannot tell their employees what to buy or not in their personal life.While an employer can set rules and guidelines for working hours,they can't dictate their whole life