Most offers have requirements for people to receive something for free. For example, an offer that gives away iPhones will require that people complete surveys or take offers to have a chance to win.
Different offers have different criteria, but it’s usually explained on the offer’s page.
About Facebook bans, they’re not related to whether the offers are free or not, but to your niche and your copy and if they violate Facebopok ad policies.
If you haven’t read Facebook ad policies yet, I suggest that you do so before you start advertising there.