I commented below with a proper answer (or my attempt at it) but basically, in each of those situations they are actual competitiors. You could replace Yahoo with google, Reddit with Digg, but you can't replace Paypal with WePay. To me at least it seems like they're school yard bullies, identified a "weak" target, one that everyone dislikes and then using that dislike to improve their own position without actually competing. It's the cheats way and for me at least, I wouldn't be happy running a business that way. If I'm going to spend my time bitching about a competitor, I'd make damn sure that I was actually a competitor.
I dunno, I don't think that's true. You can replace the search function of Yahoo with Google, but that was only one of a bunch of things Yahoo was doing. You can't fly Southwest Airlines to Tokyo, but you can on American.
As a matter of fact, Southwest Airlines limited service reach makes many of the things they tout possible. AA could never, for instance, operate with only one kind of airplane.
People love the underdog. You might think of big bad Paypal as being the weak target, but I don't think anyone else does.
