PowerShell ps1 scripts open in Notepad by default, not in PowerShell ISE. Why's that? [closed]
You almost answered your own question : This is a security measure.
The Windows PowerShell Blog article Windows PowerShell and the "PowerShell Worm" details the security features of PowerShell, which are also summed up in the article PowerShell Security :
PowerShell has been obviously designed with much more security in mind than VBScript or cmd.exe:
By default .ps1 script files are associated with Notepad. Double-clicking a script does not start it.
To reference a script in PowerShell you have to specify file path, so even if a script is called dir.ps1 typing in dir will not start it. The shortest way to reference it is .\dir.ps1.
And finally execution policies by default won’t allow you to run any scripts at all. You can lift the limitation up a bit by allowing to run scripts signed by trusted authorities.