So new users are encouraged to ask their most likely separate question as a, you know, separate question.
Question marks are a horrible indication of this kind of thing. Despite all intuitive sense, they aren't required to make a question, so it still won't catch those who just don't use it.
But importantly, there are users who may posit questions in their answers. Not a question to the site, but generally directed at the question asker. For example, if someone asks about using one kind of parser, an effective answer might begin with "Have you tried using this XML parsing method?". It may also begin with "You should try this XML parsing method", but the former version is neither invalid, nor is it treating the site like a forum. It's simply how a user might approach answering a problem, and how they want to express the answer.
Giving a warning to low reputation users for using a mere question mark would subsequently make the experience extraordinarily confusing.