Will you entrust your health to someone who doesn't have any documents supporting that he is skilled enough in dealing with health issues? I bet you would prefer a license doctor than a quack.
In Columbia, every people who wish to become a chiropractor must first seek for a license. The state will only grant those people who were able to meet the required examination and educational requirements. However, having the license does not mean that they can go out to the world and practice their skills since they can only practice their knowledge in the States where they were able to obtain the license.
On the other hand, there are also some States that give chiropractors the chance to obtain a license easily even without the need for further examination. As long as they can show a proof of their education and examination then they can be granted a license right ahead.
In general, States licensing boards would ask someone to have at least 2 years of undergraduate education although 4-year bachelor's degree is needed by some. Also, all board would ask an aspiring chiropractor to finish a 4-year program at an accredited college.
Basically, different States would possess different requirements so it is better if you can check with your local States and ask about their requirements. Chiropractor license is important because without it, you will not be able to practice your profession and may even end up in conflict with authority. So to escape these problems, be legal. Seek a license.