The specifics are unimportant. What matters is that Google was using its dominance in one market to gain an advantage in a different market. That has been illegal for a long time.
The EU doesn't do[0] Common Law (Case Law), like the US does, we have Civil Law here. It means precedence doesn't carry (as much) weight. It's a different legal system. It also means it has been written (codified) into law, unlike your previous comment states. You can read it in the article and links followed from there (it doesn't need to be written to specifically apply to websites).
Be sure to also check out the coloured map at the top :)
[0] it's a bit more subtle, distinctions have blurred a little over time, either legal system adopted some parts of the other. see the wikipedia link for more details (especially the heading "Alternatives to Common Law systems")
Of course it's relevant. There is no lock-in preventing you from visiting other Websites which are just a url or hyperlink away. When was Google Search ruled a website monopoly? What were the parameters? Who's defined the boundaries of what a search service can do and what relevant content they can provide? At what point should Google have known they've had to cede control of their own website and invest resources in giving their competitors more prominence?
What other websites need to be wary of the same ruling and at what point do they need to focus their efforts on giving their competitors virtual real-estate on their website at the expense of growing their company?
If you have more than 50% market share in a given market you're considered a monopoly. Although practically, these type of actions aren't usually taken until around 75% market share.
The US just oddly decides not to enforce it, then the EU gets shit on for being anti-American when they correctly enforce things that are illegal in both the EU and US.
