What is pass interference?

I was happy to see the USC Trojans lose today in Corvallis, OR. A couple calls by the refs baffled me, though. The first was a pass interference call when the Oregon State DB had great position and had the best chance to catch the ball. The USC receiver was behind him and reached around to break it up. If anything, the call should have been offensive pass interference. The actual call: PI against the Beavers. I have no idea why that call was made. I assume that the official just screwed up horribly, but I can't be sure.

Keeping with the USC v. OSU theme, what was the replay official thinking saying Steve Smith was in-bounds on the catch they reviewed? I'll admit, it was a close play in real-time, but you can see that he obviously landed out-of-bounds in the slow-motion replays. There was no reason not to reverse the call and rule it incomplete. But, they didn't. Yet another reason to celebrate the USC loss!

Then, in the UT v. TTU game, there was another somewhat dubious PI call. Late in the game, with Texas sitting on a 4-point lead playing keep-away from the Red Raiders, Texas QB Colt McCoy threw to the right sideline for a receiver. A Texas Tech defender was between the receiver and the underthrown ball and didn't really make a play on it. But, the Texas receiver only made enough of an effort to make contact with the defender to draw the penalty. It was a cheap call on a crappy throw that helped Texas salt away the game.

Obviously, the defender isn't supposed to hit the offensive player before someone touches the ball. But, the defensive player is supposed to at least be in the vicinity of the offensive player, right? I think we're all agreed on that. So, the defender will be on one side of the offensive player. Let's assume we have a play similar to the two I mentioned above, with the WR running along the right sideline and the defensive back between the WR and QB. If the QB throws the ball out in front of the WR and the defensive back rides him out-of-bounds or impedes his progress to the ball, I can see that being PI. But, if the ball is thrown in-line with them in front of both of them, is the defender supposed to get out of the way and let the WR step in and catch it? I don't think so. Should the defender go for the ball? In an ideal world, yeah. But, does he have an obligation to? Why can't the defender just maintain his position? If the WR makes contact, that should be offensive pass interference. The QB shouldn't make such a crappy throw. Right?