Many people - including some on HN - mistakenly equate marketing with only advertising or, more broadly, marketing communications. In truth that's only a small portion of the discipline.
The real problem is the cost. Imagine that, if people in the USA, earning it US dollars think that a text-book (http://www.amazon.com/Marketing-Management-14th-Philip-Kotle...) costing USDd$168.72 is crazy. Imagine what people in Mexico (where the average yearly income is about USD$7,300) , India, China or other development countries think.
Quite simply, a lot of the people who pirate books are not a market for the publishers simply because it is impossible to pay.
Are they entitled to get such knowledge for free? legally, they are not. Morally... it is subjective.
Get dozens of book recommendations delivered straight to your inbox every Thursday.