Why the comma? That use of a comma is weird, right? Is it just me? They used one adjective. This wasn't a drug-infested, cum-covered sex den. This was merely a drug-infested sex den.
The problem with that interpretation is that you can't have "libraries turning into drug-infested, libraries turning into sex dens." because "drug-infested" is not a noun phrase a library can turn into like "sex dens" is. I also tried misinterpreting this comma as a comma between two adjectives, which doesn't work because "sex" is not an adjective.
Maybe "Libraries turning drug-infested, into sex dens" fits your interpretation better. Does it?
I suppose, yes. The library is turning drug infested and into a sex den. The comma is replacing all the words "and into a." Which is essentially what you wrote.
I guess i was trying to point out they were two separate situations and that drug infested is not describing the sex den. I was also trying to establish that commas can replace words and phrases. Although the example I gave above only replaced one word, it would make sense they would replace other words, as well, to shorten the headline.
I guess there are actual headline specific grammatical rules that are followed. While not a comprehensive list, some of these rules include leaving out auxiliary and some joining verbs, articles, conjunctions, etc, and replacing some words with various punctuation. Apparently, the list goes on.
You're right because the hyphantion of "drug-infested" causes it to act as an adjective to describe "sex dens". But there's only one adjective so the comma is not necessary