Ex-affiliate fraud manager here.
Here's the kicker about affiliate marketing: the unethical practices exist because the system is set up in a way that there is no real incentive for mitigating fraud.
The merchant who pays the bottom line might have no idea that fraud is happening - they only see the traffic numbers going up without realizing that this was already coming in through other means (bought or organic).
The fact that the affiliate management or program might get their salary or bonus based on the size of the program (ie. traffic) does not help the situation at all. The largest affiliate in the program could be a sophisticated thief without anyone knowing any better!
There's also the issue of technical illiteracy from the operating parties - they all claim that they fight fraudulent practices, when in fact they might not even know what is affecting them specifically. This is obviously to lure in more affiliates, clueless-hopeless people who bought in (often literally in the form of SEO and other infoproducts) on the dream of passive income. They've read two-three books on white hat practices and are trying to compete with, say a mob of savvy blackhatters buying Pay Per View (read: adware) traffic and stealing the commission.
And don't get me started on the numbers.
And then you realize that to the merchant itself, it doesn't really matter, because the profit at the end is so high that throwing millions out the window still doesn't even put a dent in the side of the business.