Kind of a dumb strategy in my opinion. A lot of times I'll see a post on quora I want to share on facebook or whatever, but I don't because I know I'll just be irritating my friends when they see a "Signup to read this article" page.
However, for 1 person like you that thinks about not sharing, I'm sure there are many that will share without thinking. Moreover -on this point I'm totally sure- if you force users to signup vs you leave the option, no matter how good is your landing page, you will have more signups with the first option.
The point would be to understand how many share they lose vs how many signup they still get, but I guess the math is on their side (note: I'm assuming the most important metric here is number of signups, which is totally an assumption).
