Do people bother buying tickets? It seems like with those sort of fines it would be cheaper to simply pay the fine the few times you get caught. Unless of course Seattle have some very active ticket checks.
There are a couple of caveats. When I don't have a ticket, e.g. because I forgot my wallet, I'm on a constant lookout and the train ride becomes really stressful. If you get caught several times they might press charges against you, so it is not feasible to go without ticket on a regular basis anyway. And last but not least, I'm using a service, so I pay for it.
I used to ride without a ticket a lot when I was in school. You are right about the stress. (On the other hand, riding with a ticket afterwards is much more boring.)