I've barely touched reddit and cannot speak for them, but I do run large forums and can speak for why I silently ban.
Most forum software has the notion of "Send to Coventry".
http://en.wikipedia.org/wiki/Send_to_Coventry
I send people to coventry when I feel that they are unlikely to change their behaviour were I to inform them that their behaviour is going to lead to a permaban.
Specifically, if people are spamming and trolling, they get sent there without notification. In my experience, any spammer or troll is simply going to redouble their efforts if they become aware that they're going to be banned... but if they think they are successfully spamming and trolling then they quietly move along.
The harder bit is determining whether a spammer or troll is a spammer or troll.
There are the obvious spammers (penis enlargement, viagra, etc) and obvious trolls (Slashdot has had some famous ones). But what of someone who wants to sell something and is just over-enthusiastic, and lacking in netiquette?
That's where it's harder, but once you get to a certain size and you're dealing with perhaps hundreds of reported posts per day, then you're quite likely to be more heavy-handed, and less polite when you're working your way through the backlog.
That's when you offend someone who actually would've reacted well to just being spoken to.
So that's the why... and the problem is probably more indicative of a stretched set of moderators.