They know that observation has limits. In a legal trial, both the defense and the prosecution present what they believe to be the facts in the case. In other words, the lawyers try to prove their case. They try to present convincing arguments that show that their interpretation of the facts is the correct explanation for what happened. In science, a theory or hypothesis is the explanation for the events that have been observed.
Scientists create hypotheses to try and explain the observed behavior of the world.
If a lot of evidence is collected to support the hypothesis, then they'll probably accept the hypothesis as a good explanation, and it may, given even more testing, become an accepted theory. This is pretty much what is meant by proving something in court. If there is enough evidence, then you will be able to convince your peers that your explanation is good. It just means that you have convinced other people that the evidence supports your conclusion. And most of us probably suspect that there are guilty people who get off.