Good point. If its on the list of recognized serious adverse events for a particular vaccine all you have to show is that it occurred in a reasonable timeframe after the vaccine. If its not on the list, you have to show that it's "50% plus a hair" likely it was caused, or just barely more likely than not, which is far short of a scientific standard.
According to the people who have actually won their cases in vaccine court, this is not true. They all say that the experience was extremely adversarial, and that they had to have an absolutely airtight case. Even the lawyers agree that the Special Masters go overboard looking for ANY possible cause other than vaccines as an excuse to turn down the plaintiff.
Here is an interview with one of those lawyers: http://www.activistpost.com/2012/08/exposing-fdas-vaccine-injury-cover-up.html