Looking at that package's test results, it's not tested on Windows systems. Further investigation reveals OS-specific packages are allowed, but frowned upon if the author just didn't try for cross-platform compatibility.
The CRAN Repository Policy:
Package authors should make all reasonable efforts to provide cross-platform portable code. Packages will not normally be accepted that do not run on at least two of the major R platforms. Cases for Windows-only packages will be considered, but CRAN may not be the most appropriate place to host them.
From Writing R Extensions 1.1.1: The DESCRIPTION file:
The ‘OS_type’ field specifies the OS(es) for which the package is intended. If present, it should be one of unix or windows , and indicates that the package can only be installed on a platform with ‘.Platform$OS.type’ having that value.