"earn free" hahaha, yes. Just like when I earn free money at work in exchange for labour. Seriosly scammy name. Remove the "free" part please ata.
Technically they are "free" because you normally wouldn't "spend" anything to get them. (There are some, of course, where you have to spend money to get the nobs, but those give you hundreds.) You earn money for work because you applied for that job and that's what you spend your time doing. The free nobs are optional, whereas you must show up to work every day. (Or take leave/quit your job)