Also an individual postal worker can not make up rules as they go along. It does not matter what an individual post office location has to say; they all must comply with federal regulations.
The postal worker described in the original post may have been working under this particular section. The relevant sentences are:
"Under DMM 601.8, postmasters have the authority to determine the mailability of restricted articles and substances. A postmaster who has determined an article or substance to be nonmailable has the authority to refuse the item from mailing."
Knives are on the "restricted article" list, so it looks like the regulations do leave room for local application of the regs. However, it still seems weird that they could claim that knives are per-se unmailable.