BTW and how the faucet suppose to earn? Was always curious about that... some % of donations as "fee" or just ads selling?
One of the major reason is the "traffic".
If the site is in the list of good rankings (e.g Alexa rankings), the site is candidate for more advertisers because of this. Advertisers pays the site owners. And at the same time, the site owner can have the site registered to google adsense and approval is really quick especially if the site gain really a lots of traffics.
The faucet rewards here is considered as the owner's investment to make his traffic mission possible.