I think with this amount of money it's just a risk of funding people. Putting in strings requiring repayment if the person changes areas, when you're giving someone <$100k, is pretty heavyweight for either the NSF or Y Combinator, so they just try to pre-screen for people they think are going to use the money as intended and stick with it, and accept that some percentage of people will instead use the money to flail around for a year or two and leave. If the percentage gets too high, then I guess reconsider the screening process.
I think you're right re: clawbacks, though. I can't imagine the mental pressure were the scenario to be "you MUST stay in academia or you owe the government $XXXk". I actually know a PhD student on a (non-US) government scholarship who has a clause like that. He calls it his "slave contract".
Getting back to the original topic, I think that for any venture with a high rate of failure (startups or grad school or...), there has to be some room to allow the funded person/people to say "this isn't working" in good faith and pivot or leave. That's totally different from someone taking funding without the intention to use it properly, or just deciding they're bored or whatnot.
Every time I have taken investment, I have seen it as a commitment to see a venture through. Maybe it works, maybe it doesn't, but I am going to give it all I have. So no, I don't think, "gosh, I found something more fun to do," is a legitimate reason for walking away from a startup. If that's how somebody thinks, then they are not cut out for starting a company, because there are always points where almost anything else will be interesting or appealing.
Fellowships are different. I'm advising a new fellowship program right now and, like all education, it's seen as an opportunity to help someone learn. We expect a certain percentage of people to go on to do the thing we want people to do, but if not they'll be taking the knowledge and doing something else, which is also fine.
My concern here is really with the system. If we are pressing money upon people who aren't suited for it, then something's pretty wrong. Nobody should be "trying a startup for a bit".
For example, a portion of the people who win $30k/year National Science Foundation graduate research fellowships take a year or two of the funding, and then decide a PhD isn't for them. Some of those people, after taking $30k or $60k in PhD fellowship funding, leave grad school to start a company instead. I don't think it should be that surprising that people might go the other direction as well, taking $30k in Y Combinator funding, trying a startup for a bit, and then deciding to leave the startup and go to grad school instead.