Yes it was allowed but from the start slavery was never universally popular. Most of Americas first 90 years was spent limiting slavery. American Quakers provided the religious under pinning for Wilberforce’s fight against slavery in England.. The Anti slavery movement culminated in the civil war.
There’s tons of excuses. But I think that’s one of the big reasons America gets so much crap for slavery. We put these great ideals on paper, and then right off the bat chose not to live up to them. Add in the fact that by the time the Revolution happened a lot of Europe was already moving away from slavery and we kept it around for almost 100 years. We’re the country with the worst “look” for our slavery period.