It is pretty silly to research the laws of a planet but not the penalty for breaking the law.
"Law" is typically incomprehensible gobbledigook even to the trained policeman, and of no interest to the layman. "What is allowed and what is forbidden" is the relevant thing, and that is only a teeny weeny fraction of the broader concept of law. Surely Yar would have been studying the local rules of what can and cannot be done, and not wasting her time with meaningless studies of the law that is written around the subject in a language not intended to be comprehended.
A list of dos and don'ts would not necessarily include an attached list of sanctions. Tasha Yar never studied any "legal systems" - she studied relevant things which she defined as "customs and laws".
the existence of a death penalty NOW is often a major issue when dealing with other countries and things like extradition. Are you seriously suggesting that it will become LESS of an issue in the far future? Asking about a death penalty would be one of the first things Yar asked about, at which point she would have discovered that it was the ONLY penalty for crimes there. That's hardly a meaningless technicality of the law.
At any rate, even if she didn't study punishments, which is just a silly notion, they still didn't seem to know about penalty zones.