UAC in windows 7 is less strict than it was in Vista. Windows 7 regards Explorer as a trusted application so it will silently create the share without a UAC prompt (you can change this behavior in the Control Panel).
Your application isn't a trusted one (OK, you trust it, but the operating system does not). Probably you also don't explicitly demand administrator rights in your application. In that case Windows 7 treats your program as unprivileged and refuses access, that why the error message.
A solution could be adding a manifest to your program requesting an ExecutingLevel of requireAdministratior. See for example Demand UAC elevation for an application by adding a manifest. But then your whole application runs as administrator. There are some more granular solutions.