I work 60-65 hours on average in a Monday to Friday work week. Can my employer legally force me to work a mandatory 9 hour overtime day on Saturday even though i have already worked 20-25 hours overtime?

I left work early for personal reasons last Saturday. I was supposed to leave at 10am. I didn't get out until 11am. I had mentioned it to my supervisor prior. He told me to do what I want. That I would most likely have to take a "1/2 point". So I did. Today on Monday the manager of the plant came in and brought me into the office and handed me a paper stating that if I leave work early next Saturday that it is job abandonment. That I had not brought it up to anybody the prior Saturday. I mentioned to him that I had told my supervisor and he then stated to me that I had not brought it up to the assistant plant manager. He told me if I leave that I will get terminated. I know Pennsylvania is an at will state and that they can terminate me for whatever reason. But to force overtime on a saturday when you had already worked 60-65 hours, it's something a sweatshop would do and isn't that illegal? I work at Legget & Platt. I load trailers with carpet bedding in heat that reaches up to 120 degrees inside of the trailer.