Fair question. My answer is that the value being offered to me (emphasis on "to me") is extremely low. I have no doubt professional writers (and editors, and designers, and artists, and myriad other professionals, lest we forget them) have worked hard on these books. But my decision on how much a product is worth to me isn't based on the amount of time it took to make -- it's based on how much it's worth to me. The authors -- or, more likely, publishers -- have the freedom to offer the product to me at a particular price. I, in turn, have the freedom to buy it, or not, at that price.
So, what is this particular offering worth to me? As I said above, very little. I have some 500 unread books -- that's physical books -- sitting on my shelves. I also have a bias in favour of physical books over electronic ones, which puts the likelihood of my reading any of the ebooks in this bundle at somewhere just a tad above zero. If I was being completely rational about this, I'd have put in the price at $0, not the $1 I actually tried for. But given that my having these books on hand creates a non-zero chance of my reading them and remembering the names of the authors in the future, there is some value to the authors of my getting these books even if I pay nothing at all. If I'm prepared to pay $1 for them, though? That's a non-zero amount of money they're receiving from me. Does that not beat the alternative?
So if even $1 would be a disproportionately high cost for the value I expect to receive, why not $5? After all, the two are mighty close, right?
I suspect this is in part because I've been habituated to the $1 by the Humble Bundle -- it's what I tend to put in when the bundle comes up and I have little to no interest in any of the items on offer. In part it's because of the prices where I am -- a very rural area. $5 will get you a good meal. $1 will get you, maybe, a candy bar. $1 is also worth about 8 minutes of work at minimum wage ($7.25 in my corner of the US). $5 is about 40 minutes. But this is really me trying to retroactively figure out my immediate response, not necessarily what actually prompted that response. The bottom line is, $1 doesn't feel like a lot of money to throw away on a whimsical expense (and I do like the whole concept of pay what you can bundles), whereas $5 does.
But there is one other consideration: The whole idea of "pay what you can" kind of falls apart once you start setting hard limits on it. I initially criticised Tomely setting a hard minimum amount in a non-obvious way, but the more I think about it, the less I like the combination of "pay what you can" with a hard minimum. It's a bit like a retailer saying "Oh, it's $5, but if you want to give us extra money, go ahead."
Makes me wonder if Tomely conducted some user testing and discovered other people had the same reaction, but couldn't make the business model work without a guaranteed $5 payment, so they made it hidden. I might have done the same thing were I in their shoes. After all, the people who are looking to pay less than $5 aren't their primary focus in any case -- and everyone else would never find out about the restriction.