This is likely because currently Snoop is a WPF 3.5 application ... and so .NET 3.5 is required for it to run.
So, sorry! The only .NET Framework that you don't have installed is needed!
This is the case ... so that Snoop can Snoop both WPF 3.0/3.5 applications ... and WPF 4.0 applications.
Let me know if it is something else.