Things that many people think are illegal that actually aren't:
1. There's a widespread but incorrect belief that it's illegal for an interviewer to ask about your religion, national origin, marital status, number of children, etc. In fact, in most states, the act of asking these questions is not illegal. What is illegal is basing a hiring decision on the answers to these questions. Therefore, since the employer can't factor in your answers, there's no point in asking them and smart interviewers don't. (That said, it is illegal to ask about disabilities.)
Here's some advice on how to handle it if you're asked these questions.
2. At least once a month, I hear someone say it's illegal for employers to provide a detailed reference, or any information beyond confirming job title and dates of employment. Not true. It's legal for an employer to give a detailed reference, including a reference, as long as it's factually accurate. (That said, some companies do have policies that they won't give references, but these policies are easily gotten around; I've never had a problem obtaining a reference for a candidate, and I've checked a ton of them.)
This item is the reason for "mostly" in the title of this post; I'm a believer in honest references. But if you're worried you'll get a bad reference, here's some advice on handling it.
3. It's not illegal for your boss to be a jerk. It's unwise, but it's not illegal. The exception to this: If your boss is being a jerk to you because of your race, gender, religion, or other protected class, then you do have legal protection. But 99% of jerky bosses act like jerks because they just are, and that's legal.
4. It's not illegal to not give paid vacation or sick days. There's a very small number of jurisdictions that require a certain number of paid sick days, but the majority of people in the U.S. live in places not covered by those laws, and no state that I know of requires vacation time. Of course, most employers do offer paid vacation and sick days in order to be competitive and attract good employees -- but there's a difference between what's smart/customary and what's legal.
5. It's not illegal to reassign you to different duties or even a whole new job, assuming you don't have a contract that says otherwise.
6. It's not illegal to require you to attend work-related events outside of regular work hours, although if you're a non-exempt employee, you must be paid for it.
Disclaimer: There may be one or two states where something above is illegal (California, I'm looking at you). Some states make their own policies on this stuff, but in general, the above is true.