So what happends is:
A member in DefaultTrust, lets call him Bob for simplicity, is selling his paypal for btc, so anyone, for example Alice, who sells bitcoin to him is the one at risk not Bob. So after a successful transaction, it should be Alice who leaves a trust rating on Bob as she is running a risk as Bob(trusted paypal sellers always urge the untrusted one to go first) only goes after Alice sends first, not the other way around as Bob is not running any risk which would make him to leave a trust rating on Alice. Anyway continuing, now what Alice(and most trust farmers) can do is use virwox and incur a loss of max 10% of the amount he/she traded. Say she traded 0.1 btc(which is pretty much the average amount trust farmers use) with Bob, so now converting it back to bitcoin would take some hassle + a bit of loss max 0.01.
So has this forum's trust degraded so much that one trust feedback from a trusted member costs only 0.01 for a trust farmer?
There are many ratings given for paying back loans as well. Not a big deal. Even members have given me ratings for just a single trade which I haven't asked for but I haven't given anyone any rating inspire of being asked for it as if the person goes about scamming another member, I'll be indirectly responsible for it and I don't guarantee that any member whom I trust is actually trustworthy.
In your example, Bob being a DT members dint do wrong by leaving a positive rating but the only reason why he shouldn't have left a rating is because he is on the DT list. This makes this forum look bad because it has a list which makes DT members responsible of their ratings but when we actually try to make them justify those ratings, they don't want to argue. I've seen many newbies having ratings for just one trade with a DT member and that gives them a license to scam anyone with a higher amount as they have a green trust rating unlike others who trade with other members.