Hi Duncan,
As I see, “LES” is not about employees but someone else. (Unions, government, employers)
Pay attention on the first part of the sentence: “becouse they are worried”. Plural
If y ou say “porque se preocupa”, singular, it does not match.
Then, translating “becouse they are worried that their employees might be abused”, I would say: “porque a ELLOS LES preocupa que sus empleados puedan ser abusados”
ELLLOS = THEY
Also you could say: “porque se preocupan” Or “porque ellos se preocupan” de que sus empleados puedan ser abusados
IHIH
LEO