If I am qualified to submit a proposal and I do, why can't I ask the employer a question so I can hone my proposal more effectively?

Because you haven't paid the $200 yearly fee to go from "Basic" to "Guru" (which you can see if you click on http://www.guru.com/pro/my_profiles.aspx).

Once you do that, you can ask questions of the employer in their project descriptions.

There is an open message board on all projects posted as long as you're a paid member. It comes in handy when there is a vague project description.

If you have a free account, you'll just have to wing it and hope for the best.

You can ask as many questions as you can fit into your response. As a basic member you cannot ask a public question.

Public questions have great value to everyone except the person asking them. If I realize that a clarification is needed in order to submit the winning response, then I ask it in the response. Why should my competitors get the benefit from my question? The prospective client is hardly likely to recall that I was the bright fellow who noticed the issue. She or he will simply answer the question and now I'm in competition with others.

You appear to be in "Writing, Editing and Translation." I have hired several writers on several freelancer boards. All have been hired based on their insights, not whether they are a "Basic" or "Trans-Galactic" member. I assess their writing skills as we explore a fit. One of my first hires was a "so-so" writer with great insights. Had he shared them publicly, I still would probably have hired him, but would have been faced with 20 other responses with the same insights. Did the others merely copy, or are they also this insightful?

Seasoned professionals ask their questions privately, not publicly.