I think Dr. King refers to the Constitution and the Declaration of Independence in his speech to show that everyone should be free and treated equally because the Constitution and the Declaration of Independence have things written in them that say, for example, "All men are created equal" to show that everyone should be treated equally. Hope that helps you.
Well i reckon (see what i did there) it was because he kept causing trouble and made the kidnappers angry and kinda scared of him... in a way. they didnt want to deal with him anymore so they just gave him back