Probably because it circumvents the price of submitting your email address.

I find it odd sometimes that people don't take the same attitude towards Adblock. Sponsored links are a price too.

You don't have to submit anything.

Don't you have to submit your email address to gain access to the PDF?

Yes, and doing so also auto-subscribes you to a list. Not that I mind, but it would be nice to let someone know beforehand.

I did NOT get that. All I got was a direct link to the book.

You people are stuck on the wrong side of an A/B experiment. Tough luck~