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.