When you fund this project, it means you believe in the project & you perceive it to be a Good project. Hence, a minimum rating of 8/10 is allowed. If you believe this project deserves a rating of only 3/10, for example, you should NOT be funding it!
Why is the rating scale, then, from 0 to 10?
Once a campaign is funded and the funds are transferred to the Campaign Owner - they have the responsibility to keep posting updates on the campaign progress & telling you about how your money is being utilized. Once the updates start, YOU can then revise YOUR rating based on the quality& frequency of those updates.
At that point of time, you can choose any rating between 0 - 10 & the weighted average score of all the Funders will determine the Crowdrating score of the Campaign.