why do they do this
It is nice to be able to interact without having to commit right away. I've found that the places that I liked enough to join were the ones that I had interaction with first. For example I will reference to different sites that have a common theme, but vastly different approaches: Drawchan versus Tegaki
Tegaki was created to cross the language barriers by providing the author a means to write their thoughts out. This means that they don't have to make font accommodations for Sanscrit, Manderin, Cantonese, Russian, or English languages because the universal means for computers to interpret are right there. No language packs required. Kanji was the Target interest since there are more than five regional alterations in Japan, being able to write the Kanji instead of type setting is important.... In later days, pictorials were used by authors too.. nowadays Tegaki is a flood of images and fewer and fewer Japanese hand written notes. :( You have to register to use the services.
Drawchan was made for the author to convey an idea visually. It made it possible to visualize the meaning of written Kanji. The other reason was to compete wit other Chan's applets. Drawchan has been far more successful with their applet system, garnering the attention of RPG enthusiasts and artists looking to just chill and scribble...though the nowadays the peer pressures to inspire make many threads become elitist with nary a few incredible artists stealing the spotlight. You don't have to register for the service. You do have to pass their 'human' test though and input contact info before every publish. This set up means that the author can take the service for a test drive of sorts.
_Back to Safebooru_
Test drives, time investments, and interest is what is promoted by having an open scoring, open commenting, and open editing. This is the way Wikipedia and Youtube used to operate: individual IPaddresses would score with good intention, edit with good intention, and comment with good intention.
_the down side_ is that there is an ever increasing amount of disrespectful users. Rauchus and beligerant, an increasing number of anonymous utilizers are abusing the trust. That is what happened to Facebook so now they've decided to do away with anonymity. Go look at Facebook to see what happens when you get rid of public trust and anonymity...okay this previous statement is majorly My Arrogent Opinion.
I hope that helps a bit... Don't get too angry though, cowards are more often foolish. I voted up on everyone's comments, there's still a little good in this world!