Oh, I'm right there with you, but from talking to people I went to undergrad with, folks I met on the interview trail, etc. I know it happens. People want the school closest to their home, they want to live in a city they like, etc. And there's nothing wrong with that, and I'm sure some were "flexible" in their interests and are doing great. However I know at least one person I went to undergrad with who was open about fully intending to "game the system" to get in, so to speak, without any intentions of following through on what she said during interviews. She applied in the south because she was sick of snow, she applied to schools in "destination" cities without regard for whether the program was a fit, and wrote her applications to match the program. She's intelligent and managed to pull it off. This was an acquaintance so not anyone I've kept in touch with, but I can't imagine anyone being very happy with her in grad school.
Franklin - a lot can be screened out during the interviews, that's certainly true. But not all...so from the school's perspective, why take the risk? (Just to play devil's advocate). Faking research interests is just an example...I'm sure there's all sorts of things that could be added to that list. And of course, the line is blurry since ANY sort of application involves putting your best foot forward, and no one is going to put "I like this line work professor z does, but not this one" in a personal statement.
I'm not sure there is a good way to distinguish people who changed their minds from people who faked their interests to get in, although switching labs after your master's since your research interests shifted is vastly different from walking in the front door with a plan to go against the grain. I think what matters is that there's always a CHANCE that someones interests can shift...that's just life. Often times, it can work out fine for all involved if that person carries through on their responsibilities, etc. Realistically, not everyone is that mature about it
I'm sure my acquaintance from college is not following through on her research responsibilities...she's avoiding the lab at all costs.
Of course, students are taking a chance to...that professor might shift their research emphasis, they may up and leave to accept an administrative position at another school! You never know. So the game becomes about minimizing risk and maximizing benefits...unfortunately its not easy. Its unfair to applicants at times, its unfair to schools at times - I think its just a generally difficult process and making it completely transparent would have disastrous results, but deciding where to draw the line in between transparency and mystery is an incredibly complicated decision and not one I'd want to have to make.