Maria says Speltzer walked because he didn't acknowledge his Miranda rights during his arrest. While this misconception of the law is very common in movies and TV, it's not true. Police are not required to read a suspect his rights during his arrest, they are required to make him aware before they interrogate him. Even so, a suspect is not automatically free if his rights are not read to him. It only means his statements during interrogation are not admissible in court. Given the physical evidence they had on Speltzer and the fact he attacked police officers and already had a criminal history, he would not have been released by a judge.
Dexter gives Debra a speech about how he is the only one who should have custody and take care or Harrison. He means well but he is largely an absent father.