They can't test for 0 nic in your system because it's present in other things. Most notably tomatoes and eggplant. So there's a cutoff point, which means even if there is any contamination, it would be lost in the background. But from a good vendor, there shouldn't be even any cross contamination.
And any employer has the right to test for any substance they feel like. Some, like nicotine, are nearly impossible to completely clear because they're a normal part of many people's diets. Others are endemic to certain professions, such as if you work with large amounts of cash, you will test positive for coke at very low levels. (Say bank teller or casino worker. It's extremely small, but if the tests are sensitive enough, they will pick it up.) Alcohol, as well, is very common at low levels and some people will test positive even never having had an adult beverage if they have certain health issues, like uncontrolled diabetes.
So I wouldn't worry about it. If they're testing so tight as to pick up the fried green tomatoes you had last night, then you can play lawsuit lottery (and I would think about it) or decide if they're that obnoxious, they're not worth working for. Myself, I wouldn't work for a company that had ridiculously tight controls on legal blood substances that don't affect work performance, like caffeine, nicotine, OTC NSAIDs, triglycerides, blood sugar, and the like. I'd rather work for McD's or Radio Shack.
The current normal is that your employer owns every aspect of your life and body, but if they're that fussy about what is going on with their employee, I'm probably not going to get along with other rules they have, such as bathroom breaks or being allowed to sleep every night. (I'm getting old and the week I pulled three doubles and then a shift in 4 days wiped me out. I can't do triples any more at all.)
