Question

Why did northern congressmen refuse to seat the southerners when they came to take their seats? (hint: there are two reasons -- one moral and one practical)?

Answer 1

One of the reason is because the people that were in the Confederate were chosen to be in the House of Representatives and Senate. 
Other reason would be that they refuse to be seated with people who are capable to treat other humans with cruelty even when they possess other technological options for their production.