Doesn't have anything to do with pop supporting labels. Clients using
the POP3 protocol can only downloaded information stored in the email
headers and body. If Google wants to support labels for POP3
downloads, they need to represent that metadata in a custom mail
header.
For example, Google sticks a custom mail header (X-Gmail-Received:) in
every Gmail with what looks like a unique identifier for the message.
You can see this by clicking More Options on a message and then Show
Original. I'd assume that this is the unique identifier used to track
other information, like labels, about that message in a Google
database. However, Gmail labels are not stored in the email headers. I
presume they're stored in a Google database somewhere.
Even if Google put the labels in, say, a custom X-GMail-Labels:
header, that doesn't necessarily do you any good. X-headers are just
ignored unless the mail client you're using has been developed to use
and recognize them. You might be able to do something with filters,
depending on the client, or create an extension for a client like
Thunderbird.
All irrelevant, though, since Google's not providing the labels in the
header metadata.
--g