I think the main difference is that in Europe, you need to do your due diligence in order to get a patent filed, ie. you need to prove that your invention is a bona fide invention.
In the US, you just file the patent, and if you try to enforce it, your patent can be attacked in court, which will strike it down if it wasn't a bona fide invention. Since going to court is costly, most companies in the US just pay whatever licence fee is required of them, no matter how ridiculous the patent is.
Though from what I hear, there are legislative efforts ongoing to curb patent trolling - though they should change the way patents are filed, and much of the problem would be solved.