The problem with mechanical turk impressions/clicks is typically the country of origin, since they usually involve paying people in third-world countries a fraction of a cent which is still worth some nominal amount to them. Say you are an advertiser who runs an ecommerce site that only ships within the US. You don't want to pay for impressions/clicks from people in India because you can't actually sell to them, even if they are "real" people and not a script.
Also, there is "brand marketing" and "performance marketing." I'm not sure they have classic definitions, but generally brand marketing is based on "mindshare" and tough to quantify. This is basically any TV commercial -- Budweiser doesn't expect you to jump off your couch the second you see a Bud Light commercial, but maybe a week later when you're in the supermarket, you spot that new brand of beer and recall the commercial with Jay-Z rapping and grinding with some models, and think, "eh, maybe I'll try Bud Light Platinum, I will purchase this and consume it later because I like the idea of rapping with models," even at a subconscious level.
Performance marketing is all about ROI. You spend $100 marketing dollars, and you want to make at least $X back. Typically $X = 100 in this example, although in some cases you may want to adjust profit margins or even run at negative profit (for example, if you want to maximize revenue even at an unprofitable rate). Almost any advertiser on the internet has a performance marketing mentality, mainly because it's much easier to actually measure performance. This is why advertisers like the OP are frustrated with Facebook, because paying for bot traffic makes it extremely hard to hit their performance marketing goals.
You are right though, in that this is a huge gray area. To give two extremes, let's say you run an ecommerce site but you'd also like to monetize with ads. So you reach a deal with a shopping comparison site to display their products on your pages in your footer. If you buy AdWords to your site, and those visitors from AdWords scroll down and click on the shopping comparison ads, they are probably fine with paying you for that traffic, although you are probably paying something like $2/click. If you make a page with JUST the shopping comparison ads, and buy traffic from someone who basically infects computers with trojans and overwrites their Google search results to be your site (say at $0.01 per click), then you'll get clicks on the shopping comparison ads and make lots of money, except the shopping comparison site will realize these clicks don't lead to any actual sales for their merchants (since this is extremely poor quality traffic since all the users that end up there have basically no intent) and will kick you off. Somewhere in between adveritsing for "first page Google AdWords" traffic and "trojan/botnet" traffic is basically every other form of internet advertising, and how gray that area is, is typically completely defined by your business partners.