“Article IV, Sec 2, Clause 3 of the US Constitution acknowledged that there were slave states and that there were non slave states, and that a fugitive slave, escaping from his master in a slave state and running to a free state did not make him free.”
I think what you are saying is Massachusetts wanted to build a Constitution where slavery was safe, rare, and legal.
http://teachingamericanhistory.org/library/document/the-constitution-of-the-united-states-is-it-pro-slavery-or-anti-slavery/