Question

why did Union General Benjamin Butler Declare enslaved African American refugees to be contraband or captured war supplies?

Answer 1

He would not have to let them return to slaveholders in the South.

Answer 2

Union Army was allowed to confiscate those slaves who run away into the position of the Union Army. In other words, Union Army was not obliged to give the runaways back to their owners, when the owners wanted to have them back. After that, labor of the slaves was denied step by step in the south areas.