Writs of assistance angered colonists because they allowed for unwelcome invasions of privacy and more persecution of colonists.
When the unintentional British policy of “salutary neglect” came to an end, the British government began cracking down on American colonists.
They made laws restricting trade, imposed taxes on important documents (Stamp Act) and other items, and generally created laws benefiting the British economy but ignoring the American one.
When war broke out between the French and Indians against the Americans and British, the Brits sent soldiers to America to protect the colonies. However, when the war ended, these soldiers did not go home. Instead, they stayed in the colonies- legally allowed to live rent-free in the colonists’ homes.
Being that these soldiers were in the colonists’ homes, the British government took advantage and granted them ##Writs of Assistance##, allowing them to search their hosts’ homes for contraband: any untaxed documents or proof of smuggling or treason.
This obviously angered the colonists: not only were the British soldiers making themselves comfortable in American homes, but they were using this (unwilling) hospitality itself to incriminate colonists!