As you can read in the post, if your customers interact with your Facebook group page ("like", comment, share etc) the more your info shows up in their newsfeed. Why wouldn't you give away product to increase their exposure to your company?
Liking a Facebook page was not a requirement to enter the giveaway. If you look at the section that shows the giveaway design, the only requirement was to enter your email address. The page they see after they enter contains the social options which are optional to complete.