What you describe is tyranny of the majority. Again, law by means of superior force. An elected official is the employee of the electorate, not the master of the electorate. Bajoran officials have no more right to sign their own people into slavery as Barack Obama has to sell you as a slave to another country.
Which is why there were Bajoran Rebels who took up arms against the Occupation. Their Chosen leaders signed over the World they had legal authority over (forced, but without proof it's still totally legal) and once the sign-over was legalized the Cardassians were in charge so if the people demanded the Annexation be repealed, the Bajoran Government was no longer in a position to do so and the Cardies used their new legal authority to silence opposition by force of arms.
The natural rights of all people to life, liberty and property take precedence over any pen mark on paper. No form of slavery is ever legal in the truest sense. It just so happened that liberating Bajor would have caused a war to start.
"In the Truest sense" was ignored in favor of the Cardassians' legal rights to the planet. Which they enforced through brutality and military force.
You ARE right in that this wasn't right "In the Truest Sense" but legalities and brute force get in the way of that.