Three supporting details about why u.s. intervention in the boxer rebellion was a problem:

Answer:

1. U.S. intervention supported an already oppresive control of western nations and Japan over China.
2. U.S. intervention increased the control of the British over China, which allowed them to continue with the imposition of opium into Chinese marketplace.
3. U.S. intervention was not benefical for the United States. It was just a favour from USA to The European forces.

Explanation:

1. Boxer rebelion was looking for the removal every Western and Japanese influence  from China. They had been suffering abuses from foreign powers and wanted to gain full control of the country. U.S. intervention was supporting foreign violent actions over China.
2. Among many of the actions carried out by the foreign countries,  the opium wars, that is the military actions executed  by Great Britain to impose the trade of opium into Chinese marketplace, was one of the most representative events of the foreign countries oppresion over China. With U.S. intervention and victory over the boxers, it was just an act of backing the continuation of these abuses and more domain on China.
3. From an American point of view, there was no profit for the United States. It was just an act of support European countries and no more. The U. S. military was just part of the killing of chinese boxers for nothing.