I can actually see both sides. It boils down to 2 kinds of people that are watching streams:
1. People that watch the stream but would never buy a PPV. This has no revenue lost whatsoever for the UFC.
2. People that would buy the PPV's but don't because they get it for free online. There is revenue lost here.
However it's very hard to measure how many people are on either side. One can also argue that there are a lot of people streaming instead of pooling money together to watch it at your buddy's place, which also incurs loss for the UFC.
That said, the bottom line is that the UFC provides a service that they would like to get paid for (setting up a PPV) and their claim that if you don't pay, you can't watch it. That's what is important. I'm all for as many freedoms as possible, but when you own something, you have the final say in what you want to do with it. It's not about whether you can or should or shouldn't do something. People keep bringing up analogies and they all make sense in the posters' context. But the bottom line is, the UFC owns the content and service and so it's their decision as to how they want people to view it.
Let me use an analogy of my own:
If I decide to open an air castle for $1000 dollars per hour, would it be okay for you to sneak in and get a free hour in my castle under the claim that if you would have to pay, you would never have done it? The result is the same, you got a free experience instead of dishing out $1000 dollars and I'll feel screwed over because you took advantage of a service I provided under certain conditions without compensation.
If you made it this far reading my crap, I'm just expressing my opinion. Internet, keep on internetting