Most of the time there is a mutual respect in an employee/employer relationship, but exceptions are common.
The legal system in the USA has a Federal set of laws applying everywhere, but each state can make extra or tighter requirements, and cities and counties can offer more employee protections. There are extra laws applying to youths. There may also be contracts in place between employers and employees.
1) Legally, employment is at-will. An employer can fire or reduce or increase working hours without ay reason. The employer is only bound by law to treat people equally in terms of protected classes of people. An employer cannot deny employment BECAUSE they are of a certain ethnic or race background or other classifications defined by law such as firing an older worker because he/she is older. An employee can quit at any time without notice. There is something called unemployment insurance. Direct employees are covered to receive some government money if eligible. If out of a job through no cause of their own, they are eligible if meeting certain time on job minimums.
An employer cannot order or direct an employee to do anything illegal otherwise. No employer can order their employee to violate any government laws. No employer can order an employee to do anything considered unsafe by the government or by average people. If an employer ordered "Jump out of that airplane", an employee can legally refuse. "Go feed the lion" is typically illegal. Working facilities and conditions are defined to what is and is not allowed. Otherwise, yes, an employee is a puppet, but the puppet can just leave.
2) Yes. An employer is not required to provide a reason for dismissal. Unlawful termination does happen and can get resolved in courts or by government orders on employers. There has to be a law broken in the termination to be unlawful. For example, an employee reports illegal activities at the workplace to the government. The employer fires the employee for reporting to the government. Normally, the employee is protected by "whistle blower laws". The employer will be forced to pay the employee.
3) Depending upon US State and excluding young employees, there is no limit to working hours. There are pay laws about overtime pay. Hours can be increase or decreased as long as properly paid. US States may have exceptions. Salaries may not be lower than minimum wage set by the government, but the Federal Minimum Wage of US$7.25 per hour is very low considering inflation and old wage minimums.
In January 1980, minimum was US$3.10 per hour. Using only consumer prices, it should be $10.25 today.
If a salary is reduced significantly, but still over the minimum legal amount, an employee could quit and still remain eligible for government payments.
Workers in the USA have far less rights than in most developed countries.