Which PowerCLI version are you running? I have no problem getting the Datastore names with the latest version 5.1 R2:
https://my.vmware.com/group/vmware/get-download?downloadGroup=VSPS510-PCLI-510R2
Get-PowerCLIVersion
PowerCLI Version
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VMware vSphere PowerCLI 5.1 Release 2 build 1012425