You also couldn't patent anything that already existed in prior art, even if the prior art isn't patented. It's a good system, and it's helped to put the U.S. in the forefront of technological innovation. The PTO has handed out thousands of patents for trivial software techniques that any developer working independently would discover. Little attempt is made to determine prior art. Increasingly, the agency is granting patents for obvious techniques that wouldn't surprise a 'Net-savvy sixth grader, let alone an expert.
Software patents have their defenders, of course.
But don't believe it. And most software patents don't benefit the actual inventors. If the bullies don't get you, the parasites will. The parasites weren't interested, since nobody made much money with Linux. But all that is changing, and it's changing big-time.