Because for the first time they join the forum, they will be prompted in the main page which contains all the sections of the forum which for them is very confusing, and contains a lot of information to be taken in for their fist time (unless its an alt account, there's no problem).
Any member who is serious enough with the account he is creating here should spend enough time learning the rules first. There can be options added like making the users read all the rules at the time of sign up but people just click " I have read all the rules" and thus it will not be effective.
It is hard to distinguish between members breaking rules because they were new or unaware of the rules and ones who are doing this intentionally for some benefit. We may create a requirement for members to make an introduction here before posting anything and there others can suggest him the links to rules.