It's probably legal, if they notify you of a change in rate. I'd call it unethical, though. You share the low periods with them, why can't they share the windfall with you?
I hurt my back at work last week. Pulled a muscle rather severely. The company was good about sending me to the clinic and all that. I was put on restricted duties, which essentially means sitting on my butt doing nothing. They said if I stay home, it's a lost time injury, which jacks up their rates, so it works better for them if I'm there doing nothing. I can accept that. However, they brought around a form with change in pay plan. While I'm hourly instead of "flagging" my rate has dropped from $18.50 to $10.00. I'm not happy about it, but at least there will be no surprises on payday. Actually, the money is the least of it. I'm bored to insanity. Despite some very reasonable restrictions (no lifting heavy weights like tires) they'd rather play it safe and have me lift nothing.
About once a year they examine the various pay scales. We have salary, hourly, "flag", base plus commission, and some 100% commission, depending on job duties. If they need to motivate some people, they alter the plan, so that if you are already productive, you won't lose any, if you're above- average, you'll still do well, and if you're an underperformer you lose. They print it out and distribute it. Then you return a slip that says you understand and accept the terms and sign it. If you don't like it, time to start job hunting. Business is business after all, but at least they are open and honest about it.