I think it's more a conscious decision to default to a CSPRNG unless you know better. If you know don't know the difference, a safer default means less problems.
EDIT: it's a similar situation to general purpose allocators. If you know you can use a simple one, it's orders of magnitude less code and complexity than a general one.
The Rust stdlib doesn’t come with a CSPRNG so the point is moot. On the other hand, if you’re rolling your own crypto you deserve what you get if you don’t know the difference between CSPRNG and PRNG & you can name them differently even (e.g. rand(), insecure_rand()).
This was in response to the size of rand & dependencies, trying to be the safe default for every use. I don't have a strong opinion on whether it belongs in the stdlib.
EDIT: it's a similar situation to general purpose allocators. If you know you can use a simple one, it's orders of magnitude less code and complexity than a general one.