Why is it that some airports have them and others don't (and of course, it's always for "security reasons")?

I routinely work on written correspondence while traveling and it's nice to mail it en-route, especially if arriving at final destination after the last mail pickup or if leaving the country.

Yesterday I did LAX-SFO-MSY-ORD-IND on my way home in a pseudo-mileage-run to pick up a few extra status miles. I have friends that like to get postcards from "exotic" places, so I planned to send out postcards from LAX, SFO, and MSY.

None had mailboxes in the sterile area. UA agent at LAX understood my frustration. SFO information desk told me it was for "my protection" so somebody doesn't "blow something up" (guy made bomb/explosion motions with hands while saying this--I'm sure if I'd said those words and made those motions a LEO would have been called.) Same concept at MSY (I can forgive them; they're not a hub with connecting traffic.)

I actually left security at SFO and MSY to get to a mailbox. At SFO, I had to go all the way outside to the curb to a big blue mailbox. At MSY, there's a full post office in the building outside security that has an accessible mail slot.

I know for a fact that DEN, ORD, SEA, and PDX have sterile-area mail slots, yet all these other places think they're a "threat," and SFO is so scared they put them outdoors (of course, you could easily put a bomb into the big blue box and blow up the curb area).

I'm not asking for the ability to drop packages, just a slot that will take an envelope. And it's not like a determined terrorist couldn't leave a device in a trash can instead of a mailbox.

Whoever's making the decisions at these airports needs to undergo a serious reality check. Does anyone know if it's TSA or the airport directors that are making these particular inane policies?