As far as I know, the one making the accusations has to present proof. Am I wrong? Is it so simple that saying "Oh, by the way, I believe he stole my idea." is enough to sue someone?
No one would (Or "should" maybe) be so stupid as to go for that.
Nope, that's not how it works. You simply have to ALLEGE that the other party did something wrong via a lawsuit and then it's up to the courts to sort it out.
Intimidation lawsuits are easy to make happen as the cost of prosecution for a large company is small, where as the cost of defense for an individual is large. So all that has to happen is the big company makes legal arguments that will take the judge some time to catch on to and BLAM you've bankrupted (or worse) the defendant. Once he has no money, he's forced to settle. Intimidation lawsuit success!
That is enough to sue someone. It's not enough to win, but it's definitely enough to sue.
There's some logic behind that. To prove claims like that, you may need the power of the court to get documents or witness statements under oath. The process is know as discovery: http://en.wikipedia.org/wiki/Discovery_%28law%29
Unfortunately, it also means that the party being sued has to suddenly put up a bunch of cash to pay to defend the lawsuit. In situations where one side has a lot more money than the other, like when a corporation sues an individual, that can be ruinous. So it can be an effective method of bullying somebody.
> Unfortunately, it also means that the party being sued has to suddenly put up a bunch of cash to pay to defend the lawsuit.
In any decent legal system, you can get publicly funded lawyers if you are unable to pay for your own, and if you win, you get your costs back anyways.
No one would (Or "should" maybe) be so stupid as to go for that.