The reason why the British thought taxing the colonists of the future United States was a good idea is because the Kingdom, in the middle of the 18th century, had almost drained its treasury, spending money on numerous military campaigns, and had a lot of external debts it could not pay off. Until the colonists were obliged to only provide a share of goods they produced to the monopoly, the relationships between these two sides were relatively stable. However, the Kingdom decided to enforce the tax laws and oblige the colonists to pay with money; it helped the Kingdom stabilize, but also motivated it to further oppress the colonists. In 1764 the British Parliament forbade the colony to produce its own currency, making it have to pay in pounds, which was difficult for the colonists. Moreover, as an additional way to squeeze more profits out of the colonists, the Kingdom obliged them to submit every legal document with a special stamp on it (the so-called Stamp Act). This stamp must have been purchased for solid money as well. So, the colonists did not have enough money to pay taxes (since their local currency and incomes in general were lower than those required to easily pay off in pounds), and they could not solve a number of administrative and other questions because they could not afford purchasing the special stamp, without which any submitted documents were illegitimate. Combined with the fact that the colonists did not have any representation in the British Parliament, it can be easily seen that they were put in a powerless position.
p.s. By the way, it looks like you've placed several questions on the same topic. Check it on your side.