there is nothing free.
the reason why some of these coins are distributing their tokens for free is that they know their project is so bad that they can never hope for the minimal investment that can cover their costs so they choose the cheapest way of advertising which is spamming on the forums and spreading their token for free then hoping to get some hype to pump their useless token a little to make some money before moving on to the next useless token creation.
In most situations that is the case what you described above, but as always there are some exceptions. Blue protocol is one of them.
They airdropped their token last year and there is real team behind and a real Team and development and supporting community.
They are struggling to get on bigger exchanges as they are lacking of funds. Not all the projects want to fall under ICO regulation and
selecting the heaviest way, without raising funds.