Visibility="Private" cmdlets can be retrieved from a function

Question

I am providing my customers the ability to script with PowerShell in my application but I want to severely restrict the commands they can use.  Setting Visibility to Private for cmdlets works for the top scope, but if you wrap the command in a function
it's available again.  Why does this work?  Should the visibility still be private in the function so I can't execute the cmdlet?
PS C:\> Get-Process -name "firefox"
Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
    470      53   143336     141720   390     5.71  18380 firefox
PS C:\> Get-Command -name "Get-Process" | % { $_.Visibility = "Private" }
PS C:\> Get-Process -name "firefox"
The term 'Get-Process' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:12
+ Get-Process <<<<  -name "firefox"
    + CategoryInfo          : ObjectNotFound: (Get-Process:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException
PS C:\> function getp { Get-Process -name "firefox" }
PS C:\> getp
Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
    470      53   143336     141720   390     5.71  18380 firefox

Answer 1

I think it's by design. Imagine that getp was created by you for your users to call so they can't use the full Get-Process.
If you want to limit the ability to create functions and run scriptblocks, check out the
about_Language_Modes help topic. Try this and see if it's too restrictive (WARNING: you may have to close your PowerShell session to get it back in a usable state):
$ExecutionContext.SessionState.LanguageMode = "RestrictedLanguage"