Ian - the risk tab I beleive requires "View Finance" access, as Workfront designates the Risks as a financial area. This can be handled with the sharing.
As others have mentioned, you may be able to work around this by adding a custom tab as a replacement. Create a report of the risks, add it to a dashboard, and push that dashboard via the Layout Template in SETUP to your users. The Layout Templates can provide a lot of valuable options. We have a Project Summary tab that is a dashboard of the key project information all in one tab: Project name, Desc, PM, Sponsor, and Condition Update, a Milestone Report showing percent complete on each milestone, the active issues list, and the risk log. Note: You may have to set the reports to "run as" someone to get the risks visible becaus of the aforementioned Finance Fields limitation.