You’re thinking about books, but the content that everyone was trying to claim copyright over at the time was
news, in newspapers (broadsheets.)
It was pretty easy to “pirate” someone else’s single-spread daily newspaper — rote copying of someone else’s layout and typesetting for a single spread, split between 2–4 skilled hands, being only the work of an hour or so. If you got your fonts from the same foundry, the result might even be indistinguishable from the original!
I don't know this part for a fact, but it's pretty easy to imagine that the "news stands" of the day were vertically integrated affairs, owned by the particular newspapers themselves. But like a newsstand of today, they likely wanted to cross-sell one-another's papers as well — capture the other guy's foot-traffic, and sell them a pack of gum while they're here. But probably each newspaper wouldn't allow their competitor to buy their own paper for resale. Or, if they did, it'd be at an unprofitable markup. Bootlegging your competitor's paper, to sell in your newsstands alongside your own paper, works neatly around both problems. (And, if you're really underhanded, maybe you might even cut distribution deals to be the supplier of your competitor's paper through non-newsstand sales channels — commoditizing and thus cheapening their paper, while yours retains the exclusivity of being sold only in your newsstand.)
Fun actual fact: newspaper piracy is in part why newspapers were on the forefront of commissioning their own proprietary typefaces. They wanted a “signature” style, to make it harder for their competitors to sell pirate copies of their work — especially ones with alterations slipped in! Typefaces were effectively cryptographic signing for newspapers.
