Why did American colonists have the rights of Englishmen?
The American colonists felt they had the rights of Englishmen because their charters guaranteed them these rights and they were emigrates and descendants of emigrates from England.
The Royal Charters given to the Virginia company of London, granted the colonies formed under that Charter all the rights of Englishmen,( the citizens of England)
These charters stated that emigration from the Mother country did not cause the emigrates to lose their rights as Englishmen. Also as these emigrates had been citizens of England before they left for the American Colonies felt that they retained the rights they had enjoyed as citizens.
The colonies enjoyed self rule for 150 years undisturbed by the actions of the King or Parliament. The descendants the emigrates felt that they retained the rights of citizenship enjoyed by the original colonists.
After the seven year war global war between France and England, England acquired many new colonies and debts. These new colonies were regarded as belonging to the crown and the people living there as tenants. The old colonies of America were regarded by the king as also belonging to the crown as they had been formed by Royal Grants or Charters.
King George regarded the colonist as tenants. As tenants the colonist had only limited rights as granted by the lord. As the rights ( of Englishmen) had been given by a previous king they could be taken away by the present king. Rights that come from the government can be taken by the government.
King George took away the right of the colonists. He needed money to pay off the debts of the global war with France ( The French and Indian War in North America was part of this global war) The colonies were resistant to voluntarily paying off England's debts. So The King absolved the local parliaments installed royal governors to collect the new taxes, and forced the colonists to house, feed, pay for British soldiers to in force the taxes.
The American colonist of course disagreed that King George had the authority to take away their rights and the American revolution resulted.