And are you aware that the 1790 federal census listed no slaves in Massachusetts? Technically you are correct - slavery had not been formally outlawed but it also wasn’t practiced.
Slavery was indeed practiced in Massachusetts in 1790. If you read the old original records you will see that the owners of those individuals who had been slaves in the earlier census were later called servants. Everything was the same except slave was now called servant AND now when the ‘servants’ got too old to serve they could be released and the owner wouldn’t have to pay a bond.
Massachusetts did not want its citizens to free their slaves. They were afraid they would have to take care of them. That’s why they fined the owners who released them.