Unfortunately it does not have anything to do with idealism*.
The USA legal system was the first one to really deal with the issues of business and software law in a big way. Obvious mistakes were made. A big one was the old cold-war-style crypto export restrictions... which still plague many businesses to this day (and fortunately has been somewhat lifted for open source software).
The thing is that once you make a law (or in the case of software patents use court systems to allow it to exist) it's extraordinarily difficult to get it repealed. It's much easier and much more effective to go after a bad law or bad system before it gets established.
It's like if you have somebody bringing in a box of cockroaches into your house. If you see him coming then it's easy to stop it, once they are established then your screwed.