At a macro-level, this reasoning fails because it assumes that "good" is the equivalent of sales, and we know that literature is judged on more than just that one dimension. We also know that who gets published and who gets marketed is not a meritocracy according to any single dimension or dimensions, and that those things obviously correlate highly with sales for most authors.
This reasoning also fails because it's kind of like using a word in its own definition. This is the literary equivalent of the anthropic principle, and it makes the error of judging the cause by the effect. (It's like saying "The Mona Lisa" is a good painting because it is so popular. Or, God help us all, "The Jersey Shore" is a good show because lots of people watch it.)
The truth, though, is that in almost every instance - whether we're talking a book, an author, a band, a TV show, or whatever in our consumerist, generally democratic culture - there are two valid sides to the exchange, neither of which is being clearly expressed.
So let's parse a bit the well-known example of Stephen King's critique of Stephenie Meyer. King said Meyer couldn't write "worth a darn" but then went on to say:
"People are attracted by the stories, by the pace and in the case of Stephenie Meyer, it's very clear that she's writing to a whole generation of girls and opening up kind of a safe joining of love and sex in those books. It's very exciting and it's thrilling and it's not particularly threatening because they're not overtly sexual.
"A lot of the physical side of it is conveyed in things like the vampire will touch her forearm or run a hand over skin, and she just flushes all hot and cold. And for girls, that's a shorthand for all the feelings that they're not ready to deal with yet."Now, King's dissection of the appeal of the books may not be perceived as terribly flattering of Twilight fans (especially those older than the target demographic!), but - even if true - it is a valid reason to enjoy the books (no matter who or how old you are). And regardless of whether you agree with the exact stated reason, Meyer clearly found some way to tap into what certain people want in a story (this is the "people buy it" part, and it is undeniable). But let's separate that from the writing itself. One can enjoy books that are not especially well-written, but it's counterproductive to argue those books are well-written when they're not.
Another great example is The Bridges of Madison County. It tapped into a vein with a certain demographic (one with very little overlap with Twilight fans, beyond also being largely female) but - despite its pomposity and faux-literariness - you'd really be hard-pressed to make the case that it is a well-written, or good (in any sense but tapping into that vein successfully), book.
Thinking for just a sec about TV, people watch reality TV because it taps into their curiosity or appeals in some way. But, even if you're a fan, you'll probably fess up to morbid or prurient interest as your motivator for watching, rather than make the case that it's the complex plotlines or amazing camera work that you really love.
(The Lt. will here admit a weakness for terrible action movies on hulu.com. There's a reason they put those things up for free. I watch these horrid movies and feel the adrenaline rush, root for the hero - especially when there's a vengeance element. I enjoy them. I'll keep watching them. But they ain't good movies.)
What annoys readers (and writers) like me is not when people buy Twilight or Bridges in sufficient quantities to make them bestsellers.
And it's not when they read them as guilty pleasures.
It becomes a bit annoying when that's the only kind of stuff people read: books that tap into them at a visceral level and reinforce what they already believe or want to believe (about themselves, about the opposite sex, about love, about politics, about society, about whatever). (This is one of my problems with certain - not all, but certain - examples of so-called "women's fiction" where all the the male characters are mere window-dressing or straight out of a Bud Light commercial.)
This is the fucking Fox News of literature. But isn't art supposed to challenge us? Maybe not every time, maybe not constantly, but at least occasionally? If you're scared to read anything that isn't self-reinforcing, why bother reading? Do you really grow by reading - over and over - protagonists that are just like you, facing situations you can well relate to (even if you are unlikely to encounter them yourself) and making choices that you approve of all the time?
And this moves from a bit annoying to supremely annoying when readers then go on to claim that such books are "good," by which they mean well-written and at least kind of deep. (Nor, by the way, am I arguing that well-written and deep books necessarily resonate with many people. Whether a book is "good" is not only highly subjective, it is also highly multidimensional.)
So I think what this post boils down to is an appeal (or several) to avoid the false dichotomy people draw from instances like King-Meyer:
First, let's stop equating best-selling with good.
Second, and more importantly, let's can the use of "good" if we're talking literature, and instead say what we mean.
And finally, let's us writers seek to challenge our readers, and let's us readers seek out a challenge (or at least seek to understand about ourselves why certain books do or do not resonate with us), at least once in a while.