I'm commission based & my employer requires me to work w/o compensation while being given major pay cut but require to do same amount of work. Legal?

I am a groomer and make commission based on the grooms/baths that I do. My employer requires us to stay and release pets to owners til the last pet goes home. I may finish my work at 2:30 pm but if the last pet does not go home til 7:00pm (which is when the business closes) I am not allowed to leave til then and am not being paid to do so. They try the say it is because we have boarding dogs that get an "exit bath" and we make a commission off that, however, I AM doing the work on those pets (just like my others) and am only getting paid for the work I am doing on them, no more. We also use to make 50% commission on ALL pets (grooming, baths, and exit baths) they recently cut our commission pay to 25% on those exit baths pet and yet we are STILL required to do the same amount of work, just for less commission. This caused each one of the groomers to receive a 10,000 to 12,000 a YEAR pay CUT! Yet we are still having to do the same amount of work. How is this legal?