Eh, I wouldn't really call it "basic input validation", it's more like fuzzy presumptions of trustworthiness. You have one part of PowerShell team that is wary of deserializing ScriptBlocks into ScriptBlocks instead of strings because that could trivially lead to RCE; and then there is other part which sees nothing wrong with executing code with arbitrary semantics (e.g. Get-ItemProperty) on whatever strings are lying around in the blob.

The root of the problem is IMHO is having code with arbitrary semantics; it's undoubtedly quite handy to use but the price is the inherent uncertainty about the security implications. I mean, who is aware that if you feed Get-ItemProperty something that starts with two backslashes, then it will blindly send your credentials to whatever domain is written after those backslashes? Why is it even doing that? Oh, right, because that's how you make remote shares look like local directories.