In the United States of America, it is legal to listen to air traffic control transmission. However, this may vary based on the particular State you are in. You should also not assume that this remains the same around the world as many other countries may not allow you to listen in, or may have stricter rules.
As well as this, there may also be local and federal laws in place to restrict and prohibit what exactly is allowed and what you are allowed to do with any information you receive as a result of listening to the transmissions. There are also situations where listening is not allowed.
Around the world, these rules vary a great deal. For example, in the UK they are not allowed to listen at all! There is a law in place in the UK known as the Wireless Telegraphy Act which bans the listening of any transmission unless specifically intended for the public.