Why was the loyalty oath a very controversial issue for the Japanese-Americans at Manzanar?

The loyalty oath that the US government asked of the Japanese-American internees was controversial for a number of reasons.

In general, it was controversial because it required the Japanese-Americans to swear loyalty to a government that had not treated them as it should have treated its citizens.  It also put Japanese-Americans in a position of either (eventually) being sent back to Japan or, in essence, volunteering to go to war.

For the Wakatsuki family in specific, the loyalty oath was controversial because of what it might mean for the men in the family.  The father was not happy with the idea of his son going off to fight for the government that had interned the family.