Or, why are they not free?
Many times during my professional career I've wanted to read the underlying standard for a product or a system, and I can't ever be bothered to cough up the ~100 CHF they seem to demand. As a result I've never actually read a standard.
It doesn't seem to me to be a terrible trade-off to say that ad-hoc use via public drafts is fine, but if you need to use the actual spec for whatever reason then you're probably being paid to abide by the spec and it makes sense to charge for it.
Where I strenuously object to non-public specifications is where they're referenced in (and required by) legislation: if ignorance of the law is no excuse, the law had better be freely available.