If you’re even remotely connected with search engine optimization, you will know that in that last few years Google is aggressively fighting spam back links. First it was the Panda but there were quite a few updates after that and you can be assured many more will come in the future.
In any war there is collateral damage, similarly while most of the spam websites were reduced some legitimate sites were also hit. They do have the option of considering things and submit a request for inclusion, but sometimes the damage is already done by then.
To be fair to Google, they are providing webmasters with notifications about spam back links to get rid of them and tools to disallow them when you can’t do anything about it. But if you build links in a natural way then you limit your chances of getting such notices in the first place.
With Panda Google fought spam content and discouraged many webmasters from creating shallow content. Now Google is starting to focus on spam or questionable links. And if a large portion of your link profile consists of questionable back links then your website or blog will have a higher chance of getting penalized. The key is to have a natural looking link profile and that is why it is important that your back links also have other back links pointing to them.