Because I see a big problem with that trust system. You could setup a big group with some friends, invite more people and circle-trust each other to get infinite free SCT.
That's what I thought too, if you are big seller and you have your inner circle you can earn a lot of free SCT. Can happen something like those twitter accounts pumping followers each other
That is not so difficult to workaround, you can just sell each other virtual items, IOUs or even just 'air', with the sole purpose of boosting trust.
By the way, you can sell air in a can.
http://www.abc.net.au/news/2016-02-09/the-bizarre-business-of-air-farming/7150790