Basically, a channel like amazon prime will say go to this_URL and enter this_code.
Trouble is, lots of people don't go to exactly that URL. Some type it incorrectly (just look around this Roku forum at the 50 different ways that people type "Roku" – Rocky, Ticki, you name it, someone has typed it that way.)
Some type it into a search engine, either by going there themselves or use one conveniently built into their browser.
Some get distracted by a suggestion from an integrated search engine as they start typing.
Then, once they arrive there, they don't confirm that the domain of the URL actually matches the company they are trying to link to.
Then, the scams commence.
By the way, when I look, scam sites are usually right up at the top on Google because they are paid ads. On the other hand, Bing is way cleaner and the top link is actually Roku. Whether this is because Bing is better at scammer avoidance or because the scammers haven't thought to buy ads there, I don't know.