Why do PK_Ops classes return secure_vector for non-private information

[randombit]

Namely PK_Ops::Encryption::encrypt and PK_Ops::Signer::sign.

I was curious when this was introduced but it apparently dates back to at least 1.5.6 when I started using Monotone (later imported into git) so anyway is certainly my fault. But it doesn't make any sense and I have no idea why I chose this.