What is normal pay in U.S. for RAship in STEM field?
I only have two US experiences to offer for comparison, one from a public university and another from a private.

The public university pays it's TA/RA's ~$8000 for a 9 month stipend.  The private university pays TA/RA's $19,000 before taxes for a 9 month stipend.  Both have stipulated that the expected workload is 20 hours a week.  While the public university pays significantly less than the private university, the hours for a pure math graduate student rarely exceeded 20 hours per week (new teachers sometimes spend more prepping, but any hourly help session positions were strictly 20 hours).  Paywise, I think the rate you're being offered for a 20 hour work week is not unheard of, especially for a public university.  However, being told you actually have to work 40 hour weeks means you're working for a rate less than the legal limit.  That certainly counts as getting screwed in my book.

Personally, I would say run like hell.  I know that academia has a history of overworking/underpaying graduate students, but I would find an alternative arrangement even if it means finding another school to attend or delaying a year.  Another consideration is that working 40 hours a week makes it difficult to actually do well in your studies and it wouldn't be ridiculous to consider picking up a side job just to make ends meet (meaning you're potentially working 60+ hours a week).  If you enter this arrangement, I suspect you'll be back here asking how to deal with burn out or how to recover from a mental breakdown from stress.  If you seriously want to be successful in your program, you need to find something better.