法律谘詢:雇主拒付工資怎麽辦

(posting this for roommate):
I am a chinese lady in my 50's, I came to US on visitor visa, I worked for an family for 3 weeks as babysitter and had to quit the job for personal reason , after I told the family they were mad and refuse to pay me the salary (about $800),when I try to fight for it, they threatened me of being working without permit so I can't sue them.

The couple are Canadian and they are in US on H1 visa, I want to know is it leagal for them to hire someone without work permit? I feel very bad and I don't want them to get away easily.

anyone who knows about the immigration law please help, very appreciate.