Why does StrictYAML not parse direct representations of Python objects?
Regular YAML allows the direct representation of Python objects.
If we load this YAML formatted string in, we get a user object. This was how YAML was intended to work since it allows the ability to write objects to and read them from, say, a database.
By itself, this behavior isn't necessarily capable of enacting a successful attack, so not all code that parses untrusted YAML is insecure, but it can be used, especially in conjunction with metaprogramming to execute arbitrary code on your system.
This shares a lot in common with the pickle module's behavior, which is why its use with untrusted input is strongly recommended against in the Python docs.
This anti-feature led to Ruby on Rails' spectacular security fail.