Freebies or Giveaways, most gambling sites are offering this thing. Their only goal having this is to attract players/gamblers, it is obviously for newcomers who will look into that but I don't think it was a good idea and I don't rely on such promotion but to consider most on their legitimacy even they can't offer those freebies, free spins.
Well, I hope OP could make a list of only those sites that are worth trusting, not because they have such attractive offerings.
That giveaway is only working in the first launch of gambling sites but after they got many players they will stop it . instead of giveaway they make an event and to be part of the event, you need to use your own money and they will give bonuses to new players that they can use to play unlike giveaway bonus has a required wager before you are able to widraw it.