Look, I wouldn't do it, because I know that that's what they're going to do. For one thing, it's just going to waste my time. But what if you went without preexisting knowledge of what they were likely to do and were surprised that they did exactly what you, the inexpert person, did.

What if you had a problem with your car where, let's say, the trunk wouldn't stay closed? You put a bungee cord on it so it doesn't fly open and take it to the mechanic. You hang around for an hour or so while they fix your trunk. You pay them your $80 or whatever. Then you get home and find that they've removed your bungee cord and replaced it with their own bungee cord. Would you be satisfied? Would you be arguing that demanding your money back would be stealing?

When you take your car to a lube shop, you are paying for the fact that they can do it faster, they have all of the parts on hand, you're not the one getting grimy, you don't have to crawl under your car, etc. When you go to H&R Block, the only advantage you get is that your fingers don't have to touch the keyboard; everything else that has to be done is still done by you and you alone. In addition, it takes longer because you have to drive there and back, and a verbal interview takes longer than reading. Not to mention that that one document you forgot is no longer in the next room, but across town.

I'm not suggesting that you go and waste their time; that would be stealing, but I think there's a reasonable expectation that a company running a services business provides some advantage to you doing it yourself, and with tax preparers doing relatively simple returns, that's just not true. If you go with that expectation and it is not met, I don't think being unsatisfied is at all unreasonable. And they explicitly state that if you are not satisfied, you don't pay. I'm sure they also don't give you the forms in those circumstances. It's not like you got something for nothing, other than the knowledge that these "experts" are no better than you are.