First of all, you don't know who to block until you've already been charged for the traffic they sent. So they're spending your money first, and then making you spend your time (read: money) putting up barricades to stop them from screwing you.
Let's use an example that's not-too-loosely based on a real story.
Let's say shittyclicks.com sends you 100 visitors in June and none convert. Their time-on-site is very low and you can just tell that the clicks were... shitty.
So you add shittyclicks.com to your list of blocked domains in YSM and go on your merry way.
At the end of July, you go through your logs and see you're still getting traffic from shittyclicks.com, so you call up YSM. On the phone with YSM, you talk them into opening a case for an investigation into this problem. They tell you that you'll have an answer in 15 business days.
16 business days later you haven't heard anything. You call back and reference the case number, only to be told that it's still under investigation. You complain that it was supposed to have been resolved by now, and in response, the YSM rep offers to "escalate" the case to another investigation team.
6 business days later you call back and they still have nothing to tell you. You inform the rep on the phone that you just want your ads to show on yahoo.com, and that a lot of advertisers feel the same way and are taking their money to Google instead. The rep actually laughs at you and says "that doesn't matter". She promises to forward your concerns to your dedicated account manager and have them email or call you before the end of the day.
5 business days later, without ever hearing from your "dedicated" account manager, you call up and get transferred to them directly. They tell you that the investigation concluded today and that your account has been credited $8. Keep in mind that you're paying $1.50 per click on average, and that you're still getting traffic from shittyclicks.com even as you're on the phone trying to figure out what you'll buy with your new $8 credit.
The conversation that follows goes something like this...
You: The amount of clicks I received, multiplied by my avg CPC, is WAAAAAY above $8.
YSM: Yes, we only billed you $8 though, which we are refunding.
You: Is there a report in my account where I can see that?
YSM: No.
You: Ok so you want me to just trust you on that one. How about the clicks I'm getting right now from shittyclicks.com?
YSM: We won't be billing you for those.
You: Is there a report in my account where I can see that?
YSM: No.
You: So I should just trust you on that one too. How about stopping the traffic? What if I don't want my company's name coming up on a site that may be using blackhat methods to drive traffic? What about the clicks I received from them during the course of your 30-business-day investigation?
YSM: If you like, I can open another case for you and have someone look into that...