It doesn't matter if they did something illegal. Under the interstate commerce act the Federal Government most definitely has the constitutional authority to regulate internet search if they choose to.
The government recently exercised that power to regulate ISPs. The FCC didn't accuse those companies of acting illegally.
>Why would they expose themselves to such scrutiny?
To make more money. If they think they can get away with it, of course they will abuse their monopoly. Show me a monopoly that hasn't done this.
>I thought you didn't need a huge market share to be anticompetitive. Apple didn't have a huge market share in ebooks when it colluded with the publishers.
Their isn't a magical market share that allows anticompetiveness. The key is to look at the potential harm that Google's monopoly allows. As a society we do it all the time. The constitution says we have a right to bear arms, but we still regulate weapons of mass destruction because the potential for harm is so great.