The forcible removal of a man on a United Airlines flight highlights the fact that passengers are largely at the mercy of airlines when flights get overbooked.

Overbooking flights then rebooking passengers to free up space is legal—the Department of Transportation has general guidelines about overbooked flights and how to compensate barred passengers. Europe has a different set of rules.

U.S. airlines bumped 40,000 passengers last year, not counting those who volunteered to give up their seats. United booted 3,765.

"Airline contracts of carriage state that seats are not guaranteed, and are written for the airline’s convenience not the passengers," George Hobica, founder and president of Airfarewatchdog.com, said of U.S. guidelines. "In this case the passenger had no 'legal' rights."

But Hobica said that "what’s so odd about this incident is that even if the flight was oversold to paying passengers, it appears from reports so far that this passenger was evicted from the plane to accommodate non-revenue United Express employees."

Travel lawyer Alexander Anolik wrote about involuntary removal for the American Bar Association in 2013 and said that passengers cannot sue an airline, but must rely on the Department of Transportation to enforce consumer protection regulations.

According to the Department of Transportation, "the most effective way to reduce the risk of being bumped is to get to the airport early." The department's website explains that bumped passengers are typically those who check in last.

Airlines will also usually bump people flying on the cheapest tickets because the required compensation will be lower. Carriers have other rules, too. United says that when deciding who gets bumped, it considers how long it will take for passengers to reach their destination on a later flight, it won't break up a family group, and won't bump minors who are traveling alone.

Airlines are most likely to oversell flights during busy travel periods such as spring break and the summer-vacation season, but bumping can happen any time there is bad weather that causes some flights to be canceled.

Federal rules spell out how much the airline must pay each passenger and airlines must give bumped passengers a written statement that explains their compensation rights.

If the passenger will arrive between one and two hours later than planned — or between one and four hours for an international flight — the airline must pay the passenger twice the amount of the one-way fare to his destination, up to $675.

If the passenger will be delayed more than two hours — or four hours for international flights — the airline must pay him four times the one-way fare, up to $1,350.

Anolik noted that the legality of overbooking flights was established by a U.S. Supreme Court case between Ralph Nader and Allegheny Airlines in 1976. From that case, it established "a precedent allowing them to overbook so long as they give passengers sufficient notice."

This again raises questions about the notice the passenger received on Sunday's Chicago flight.

United’s online "Contract of Carriage" says that “if a flight is oversold, no one may be denied boarding against his/her will until UA or other carrier personnel first ask for volunteers who will give up their reservations willingly in exchange for compensation as determined by UA.”

However, it is unclear if this rule is applicable to passengers onboard who have already been cleared for boarding. NBC has reached out to United for comment.

United Airlines spokesman Chris Hobart told The Associated Press airline employees were "following the right procedures" for the flight, which United has described as overbooked, when they called police who then dragged a man off a plane at Chicago's O'Hare International Airport.

However, the flight was at capacity, and only overbooked when the airline realized employees needed four seats on board, according to the airline's description of the events.

Other major airlines—Delta, Southwest and Alaska—offer similar explanations about their overbooking policies. In the event that a passenger is involuntarily denied boarding, they will be compensated in conjunction with the length of their delay and put on the next possible flight. Delta’s site says that if no flight is available, they will even coordinate with another airline.

Like United, these policies say that a passenger may be denied boarding, but do not specify if that includes forcing a passenger off a flight once he or she is onboard the plane.

United CEO Oscar Munoz said in a statement that the company is “moving with a sense of urgency to work with the authorities” to conduct a review of what happened. He also said they are reaching out to the removed passenger directly. One officer involved has been placed on leave, the Chicago Aviation Department said Monday.

Not everyone gets upset about being bumped. Some savvy travelers see oversold flights as an opportunity. They'll give up their seats if the airline makes a sweet enough offer. Some check their flight's seating chart ahead of time to see if it's sold out. If you aim to be bumped, sit near the gate agent's desk so you can pounce before other passengers take that offer of travel vouchers, gift cards, and sometimes cash. If offered a spot on a later flight, make sure it's a confirmed seat. And don't check a bag.

--Shannon Ho contributed to this report