There is no law enforcement in some countries, they can do whatever they want to. In my country for example I could sue the shit out of them, because they are hurting my business on purpose and I am losing money cuz of them.
In any country, including the US where suing is very common, it's extremely hard to sue others on negative SEO. Actually, scratch that, you can NOT. Because no lawyer can prove that this actually harms your business without Google disclosing their algorithms first and that will NEVER happen. So, the best thing you can do is protect against such attacks.
Google actually offers a lot of protection built-in. For example, nowadays, if you blast your competitors with a massive amount of shitty unrelated links (like porn, pharma, gambling, etc...), nothing will happen. Those links will get a weight of almost zero, and if anything, they might help you rank higher, not lower.
However, if you blast your competitors with exact-match (or even partial-match) related links, you might take them down for over-optimization. Google cannot protect you against this unfortunately. It's a side effect of the Panda/Penguin algorithms that can only be avoided by you. In order to protect yourself against these more sophisticated attacks, follow the phpbuilt on-site SEO guide. It also teaches you this and it works (tried & tested multiple times on different site/niches).