1. They don't try to influence society in any way. They don't take sides on any issue, but just play on the opinions of whomever the novel is targeted to. This is what allows them to be bestsellers.
2. They are not written to last. They only really work on the specific culture they're written for and, after that, become lost in unimportance or become period fiction.
The term "trashy novel," then, is really an inappropriate term for these books. It isn't that they're trashy, they're just written in a way that the masses can understand and enjoy them.
This does not mean they are written badly. Let me reiterate: yes, there are some very poorly written books out there, but there are also some excellently written ones.
And, here's the other thing - it's okay to read for pleasure once in a while. In fact, it's okay to read for pleasure often! What are we reading for, anyway? Do we read to be enlightened? Do we read for philosophical insights in the world? Do we read to learn? Do we read to be influenced or convinced of a certain opinion? Perhaps. But another great reason to read is because it's enjoyable.
I used to be of the belief that, well, I'm an English major and English majors don't read anything that isn't highfalutin' literature. They certainly don't read that trashy stuff you see sold in airports and that people read on the subways. Heavens, no!
We do read that stuff! In fact, as a writer, if you want to make money writing, that's the sort of stuff you've got to write!
Reading for pleasure is not a crime. In fact, in my opinion, it's the number one reason why people should read, in the first place. Then, unbeknownst to them, they'll also learn (if they're reading a trashy novel of good quality) vocabulary, grammar, sentence structure, syntax, and all those other lovely Englishy things! It just happens when you read enough. It all becomes so natural because your brain drinks in the styles of what you've read and becomes accustomed to the correct way of doing things. And, just because these novels aren't literary doesn't mean they don't still comment on society. There are great things to be learned from trashy novels. It's like looking at the world in a mirror, sometimes, and noticing everything about it - the good and the bad.
If the only appropriate novels to read were literary ones, almost no one would be reading.
And that, folks, would be one of the greater tragedies of the world.