China had been purchasing opium abroad from other countries for several centuries. The importation of opium was banned in the 1700s, but opium was still legal for sale in China. The effect was that foreigners were barred from participating in the business. Chinese merchants smuggled British opium into China and sold it as Chinese opium.
Chinese people definitely wanted British opium and Chinese merchants definitely wanted to sell it to them. If opium had been illegal, you might have a point; but it was legal for sale in China until 1799.