Answer: Because both are forced upon the said people.
Step-by-step explanation:
In other words, Thomas Paine said this because taxation without representation was the people had so say in the British Government's decisions. This is the same for slavery, where the slaves were forced to work, without anyway to express their disagreement or pain. The taxation was forced on the colonists, and the brutal work of slavery was forced on the indentured servants+ African Americans.