The British had placed a tax on all paper goods and legal documents. The colonist found this to be unfair because the colonist had no say in the British Parliament (taxation without representation).
The seal law, passed in 1765 by the British Parliament, was a law that required documents circulating within the territory of the colonies to use British stamps, ie limiting the freedom of the settlers. The main reason American settlers considered the law unfair was because they had no representation in the British parliament that approved the tax, as the settlers had no decision-making power in the British parliament.